AthlonXP Underclocking for a Low-Power Fix
 

It's a lot easier than you think. A few simple tweaks can transform an AthlonXP with the right motherboard into a mobile version that throttles down to a clock speed of 300 MHz for a range of applications that do not need 1 GHz and more horsepower. We also detail why our power-consumption reducing hacks do not apply to the Pentium 4.

http://www.tomshardware.com/cpu/20041001/index.html