In earlier versions of the BIOS, Dell had the fan on almost 100% of the time. People gave Dell "quality feedback" asking Dell to keep the laptop quiet for their business / school / home office use, and Dell took their advice.

Dell's latest BIOS for your laptop features an improvement which keeps your laptop quieter, longer. They do this by raising the temperature at which your laptop must get to before the fan turns on. It may seem like your fan never turns on, but the truth of the matter is your BIOS is telling your fan not to turn on because it simply is not hot enough yet. Rest assured, when your laptop gets too hot the fan will kick on.