if expanding you compuer RAM is out of reach due to limited money......try using a USB Drive as Virtual Memory Drive....  Basically Windows uses Virtual memory when your actual RAM Get full. this is when you see a hight rate of flashing on the Hard Drive access indicator LED, and your computer is still running slow. 

if you have a spare 1 gb or gb USB drive that you plan to trash..."do not" ... use that instead as a Page file Drive (Virtual Memory) you will see a significant change in computer Speed and responce... a lot quiter too... you will still see hard drive light flashing. but as much when the computer is Thrashing your drive......which is one of the causes of Hard Drive Failure

1gb or more is for windows XP,  you will need at least 4 gb or more for Windows Vista

google hard drive thrashing to get a good definition

Hard Drive Thrashing will damage your hard drive to a point you will not be able to access back your information without the help of Hard Drive Recovery companies. which cost hundreds of Dollars compared to a $39.00 flash drive