When your PC begins heating up, the System Cooling Policy determines whether to a) slow the processor, or b) speed up the fans. In Passive mode, it slows the processor; in active mode, it speeds up the fans.
Click Start->Control Panel->Hardware and Sound->Power Options->
Click: Change plan settings (next to currently selected plan)
Click: Change advanced power settings
In new window, scroll down to and expand +Processor power managment
Expand +System cooling policy, and select Active