Hardware Required

PC Case
Floppy Disk Drive
Hard Drive
CD-ROM Drive
Processor
Processor Cooling Fan
Motherboard
Memory Modules
Power Supply
Video Card
Keyboard & Mouse
To build a basic PC, you will need at least a motherboard, a memory module, a processor with cooling fan, a power supply, a hard drive, a floppy drive, a video card and a CD-ROM.

Most electrically sensitive hardware comes in a static bag which is designed to protect the electronics from static electricity shock. Leave your hardware in these bags until you are ready to install them.

System Disk
Device Drivers (these usually come with the hardware above)
Operating System

Cables and Miscellaneous

Drive cables
Motherboard spacers (usually come with the motherboard, but are used to space the motherboard up off the mounting plate)
Screws (usually a whole pile of screws will come with your PC's case, but if you are using a case you happened to have around, you will need to collect some screws)
Power cords (for both your PC and your monitor. They usually come with the hardware when you buy it, of course)
CPU Cooling Compound
 Remove Case Cover
This is a very easy step. Basically, you are just taking the cover off your new case.Each case is a little different in how it comes apart. There are almost as many designs as there are companies that make them. - take your time. open case

Now that the case is open, now is a good time to go through the screw supply provided with the case. These are usually held in a small plastic bag nestled inside the case. Inside this bag you should find:

Chassis screws - this is the type used to tighten down cards, etc.
Smaller screws- just like the chassis screws, just with a smaller diameter. It is used to fasten the motherboard in.
Standoffs - these are screws that are used to hold the motherboard about 1/8" from the motherboard mounting plate. Their ends have a threaded opening in them that accept the smaller chassis screws. If you have an AT case, you may find small white standoffs. These serve the same function as the metal standoff, but are simply punched through the board and slid into slots on the case. They are rather clumsy to use compared to the metal standoffs, but they get the job done. Lastly, some cases use small metal clip-looking stand-offs. They are pinched together and slipped into small rectangular holes in the motherboard mounting plate and they snap in. These are, too, a bit awkward.
Washers. These are typically small, loose washers, not the metal kind you’ve seen in your toolbox. These will be used to cushion your motherboard from the screws you will be using to hold it in. Some motherboards have metal plates around the holes to keep the screws from shorting the circuitry, and in this case, washers are not necessary and may not be included.
Now, verify a few things have been done, if they need to be done.

Clean Case - if used ,Install Feet - These are little tabs inserted into holes at the bottom of the case. The case sits on these tabs when on your desk.

Install Power Supply
If the power supply is not already installed, you will need to install it now. Here's how:

Take the power supply unit and line it up for placement into the PC case. The fan should blow toward the rear and the wires should face forward.
Insert the PSU into the case. Sometimes this takes a little maneuvering to get it into position. 

Install the CPU
The next step is to install the processor onto the motherboard. Now, at this point, the motherboard should just be sitting on your work space, preferably inside of the static protection bag in which it came. Over the next few steps, we will be installing some hardware onto the motherboard before it is installed into the case. The reason is that, in most cases, it is a LOT easier to do this with the motherboard out the case than with the motherboard in the case. The chief reason being room to maneuver your big hands.

Installing the CPU is a pretty straight-forward process. The real risk is to the CPU. Doing this step too fast or carelessly can result in damage to the processor. Therefore, don't get nervous. It is an easy step, but do it with care.

There are several common interfaces for CPU's today: Intel currently uses Socket T (775) and AMD uses Socket 939.. The ZIF socket opens and closes using a small lever. When the lever is down, the CPU is locked into place. When in the upright position, the processor is loose and can either be installed or removed.

All modern systems make use of the zero-insertion force (ZIF) socket. Therefore, this procedure is relevant with that setup.

Check the pins. Turn the chip over and inspect the pins. Are they bent? They should all stick straight up. If many of them are bent, then it is best to request a replacement processor. If only a couple are bent and the bend is not that much, then you may be able to use a screwdriver to gently bend the pins back into place. Do so VERY carefully.
When done, there should be basically no gap between the bottom of the processor and the socket.
Close ZIF Socket. Just close the lever. You will probably feel some resistance. This is normal and it should close anyway. If you really need to lean on it, though, check to be sure the chip is installed correctly. When down, make sure the lever snaps into place.
Some retail processors come with the heat sink and fan already attached to the CPU, in which case you will need to attach the CPU fan to the socket at the same time as you close the ZIF socket. After you lock the CPU into place, take the retention clips on either side of the CPU fan  and push them down until each side clips over the tabs on either side of the socket. Sometimes it takes using a screwdriver as leverage to be able to get the retention clips out and over the tabs, but if you do this be very careful not to slip and jab your motherboard with the screwdriver.

 Install Heat Sink/Fan
Today's processors are running quite hot. Advancements are being made to make them run cooler at higher speeds, but the importance of a high quality heat sink and fan cannot be overstated. PCs that are not properly cooled can be quite unstable, or at its worse, it may not even boot properly.

It used to be that you could attach a heat sink and fan to your processor directly and not worry about it. Today, though, processors run too hot to do this and expect a reliable PC. One must use heat sink compound to seal the gap between the heat sink and the top of the processor.

Attach the fan to the heat sink. This step is almost always already done for you, but if not, you must do it yourself. This is done using the four screws that came with the CPU fan.
Clean the top of the processor. Using a lint-free cloth and isopropyl alcohol (or some other non-abrasive cleaning solution), ensure that the surface of the processor is clean and free of dust and finger oil. Do the same to the bottom of the heat sink. Pay attention to the note above on heat pads if your heat sink had or has a rubber heat pad.
If you are using a cooling shim, place it onto the top of the processor now. Not all processors require shims.
Attach The Heat Sink. Place the heat sink/fan combo squarely on top of the processor, pressing down lightly. Do not do any twisting as you install the heat sink. Press down firmly, but straight down so as to preserve the heat sink compound layer you just applied.
Double-Check. No compound should have oozed out from the sides. If it did then you applied too much and need to remove the HSF, clean both the heatsink and CPU and start over.
Attach fan to power source. Unless your CPU fan is powered via a standard power supply plug, it is probably powered by a wire attached to a 3-pin power lead on the motherboard itself. You can attach this now It should be pretty easy and obvious.

Install Memory
You should now install your memory modules. For the purposes of this step, we are assuming that you have already chosen the appropriate memory for your PC in Step 1. So, we will jump right into installing the memory. It is important that you consult the manual for your motherboard to see about any particular sequences in which memory should be installed on your board. The installation of module modules is basically the same regardless of type, even though each module type looks a little different.

Ground yourself by touching an unpainted metal object. This will discharge any built up static electricity in your body.
Pick up the memory module by its edges.
Lock the module in place.

 Configure the Motherboard
At this point, your have your processor, heat sink and fan and your memory installed onto your motherboard. In most cases you are now ready to install your motherboard into the case. In some cases, however, it is necessary to do a little configuration on your motherboard beforehand. It is easier to do this with the motherboard sitting outside of the case.

The settings that may need to be configured are:

CPU Speed
Bus Speed
CPU Voltage Setting
Most motherboards in use today make use of the CMOS settings to configure these options. In this case, you can skip this step because you will need to wait until your new PC is powered up in order to configure these options. If, though, you are using an older motherboard in which these settings are controlled via the use of jumpers, then we need to tackle this here.  manuals list the settings next to a list of commonly used CPU's, showing the common settings for each. While this format is easier for the end user for easy setup, it is tougher if you like increased control of the settings, for overclocking for example.

When playing with the board, be careful with it. Avoid placing the board on the static bag it came in, as this can cause an electro-static shock to build up, which may very well fry the motherboard. Always place the board on a flat surface, wooden desks work best, not carpet or anything like that. And always ground yourself before handling the board. When handling the board, handle it by the edges only when at all possible.

Double-Check all of your own work. Better safe than sorry.

 Install the Motherboard
Now you need to install the motherboard into the case. If you’re following this tutorial, the CPU, fan and memory will already be installed onto the motherboard, so you will be installing this whole setup into the case now.

Connect Motherboard To Case
Now it is time to begin connecting your newly installed motherboard to the various wires of your case as well as it's power source.

 If you have a good case, each connector will be labeled to tell you what case feature it leads to. If this isn't the case, consult the manual. The motherboard will probably be labeled accordingly. Just plug in the plug. If your system has separate plugs for each, connect them separately.
Consult your manual.Connect the PC speaker.Double-Check your work, as always. Note that if an LED does not light up, its case connector needs to be flipped 180 degrees.
 Install Floppy Drive
Connect the power supply to the floppy drive. 
 Configure the Hard Drive & CD-ROM
Before physically installing your hard drive or CD-ROM, it is easier to configure them outside of the case. Configuring them involves changing jumpers, and doing this within the confines of the case can be quite difficult sometimes. You can attach any IDE device onto your IDE channels in any order. But, it is recommend you use IDE1 for your hard drives and IDE2 for your CD drives. It is always best to keep the CD drives on a separate channel from the hard drives.Configuring these drives is very easy. Often the jumper settings are printed on the top of the drive itself. If you are using SATA Hard drives, you are in luck. Because each SATA drive uses its own channel, there is no need for jumpers or worrying about master/slave relationships. Install Drive
Whether you are installing a CD-ROM, a DVD-ROM, a CD-R/RW, or even a DVD-R/RW, each of these drives install the same way. The installation is quite simple.

Install The Video Card
You must have a video card installed in order to complete this tutorial so that you can see the output from your new PC once you turn it on. Installing a video card (or any expansion card for that matter) is incredibly straight-forward and easy.

Now, you are about ready to turn your new PC on for the first time. But before we do so we need to give everything the once over and make sure we didn't miss something. So, with a flashlight, check all of your work. It is better to "waste" the time than to engage in wasted time trying to track down why the system will not boot.

Drives properly connected to the power supply ?
CPU fan attached to the power supply or to the power connector on the motherboard
If this is an older AT machine, ensure the P8 and P9 main power connectors are installed properly, with black wires in middle.
The 110/220 volt switch on the back of the power supply is configured properly for your area
Ribbon cables attached correctly and securely. If using rounded cables, the arrow on the connectors indicating Pin 1 should be aligned with Pin 1 on all drive connections. On gray cables, ensure the red edge is aligned with Pin 1.
All connections tight, no connectors off by one set of pins
If there are any key motherboard settings which are jumper-controlled, ensure these settings are correct
No wires or ribbon cables protruding into fan blades
Power switch connector on ATX machines properly connected to the PWR_SW pins on the motherboard. If this is not properly done, the machine may not even turn on when the switch is pressed.
 Initial Boot-Up
It is time for the moment of truth. To see if this thing works!!

Let's get everything connected and prepared to turn it on:

Connect your mouse and keyboard
Connect your monitor to the video card and connect the power cord to the monitor.
Connect the power cord to your power supply on the PC itself.
Okay, now for boot up time!

Stick your system disk into the A: drive. You should have prepared or gotten a system disk in the first step of this tutorial. If you are using a bootable CD-ROM (as is the case with Windows XP) then just stand by on this because the CD drive will not be openable until the power is on.
Turn your monitor on and let it heat up a few seconds before proceeding. Heating it up for a few seconds ensures you don't miss any potential error messages because the CRT tube is not ready to display an image.
Before hitting the power switch, take note of what to expect. If you notice something awry right away, you may need to quickly turn the PC back off. Here's what to look for:
a. The power LED should turn on
b. The CPU and PSU fans should start spinning
c. The hard drive should power up.
d. You will see the video BIOS screen first, then you will see the BIOS screen and it will proceed to count the memory.
e. You may hear one beep from the PC speaker. It is possible you will get more than one beep, which indicates an error which we will address.
f. You may also get a "CMOS checksum error" or another error saying the CMOS or time isn't set.
g. Know what key(s) to hit to enter CMOS setup. This will be shown on the bottom of the screen usually during the memory count. You will want to press the stated key combination to enter setup immediately because CMOS setup is the next step.
h. If you hear any weird sounds such as grinding, scraping, or loud whining, be ready to turn the system off immediately.
i. Keep in mind that if you miss the stated sequence to enter the CMOS setup before the boot sequence moves on, there is nothing wrong with just hitting the reset button and rebooting until you do catch what it is. It will not hurt your PC to reset it immediately or turn it off quickly if you notice a problem.
Press the power switch. If it powers up, observe the system closely. As soon as the BIOS screen appears, press the appropriate key(s) and enter CMOS setup. The correct key combination should be visible at the bottom of the screen. Sometimes it pops by too quickly for you to see which keys to press. No problem. Don't hesitate to just hit reset and try again, as stated above.
If everything started up as expected and you successfully got into the CMOS setup screen, just let it sit there while you take out a flash light and inspect the system as it is running. Make sure all the fans are running. Make sure all the fans are operating smoothly and not generating any strange noises. Make sure the case power LED is on. Make sure the floppy drive light is not stuck on. If it is, the ribbon cable is not properly aligned with Pin 1 and you will need to turn the PC back off and flip it around. If any of the fans are not spinning, turn the PC back off immediately and plug the fan in. You do not want to run the PC for long without fans running, especially the CPU fan.
 Configure The BIOS
Now, your new PC should be up and running and you should be staring at the BIOS setup screen.

Assuming you handled that properly, the PC should be up and running and you are either sitting at a command prompt or some other screen dictated by your bootable CD or diskette.

At this point, your new PC is now ready to go! Next, you can begin installing your software and customizing your new computer to suit your needs.

Congratulations. Enjoy your new PC!