On laptops using Intel CPUs, Windows XP uses Intel's SpeedStep technology to decrease the speed that the CPU runs. It is supposed to do this when the laptop is running on battery power, because it significantly increases the run-time of the laptop. When the laptop is running on utility power, it is supposed to increase the CPU speed back to the max - but it often leaves it stuck at the lower speed. Worst of all - there is no manual control within Windows to set the CPU speed back up to the max.
Fortunately, there is a utility that will allow you precise control over the speed of your CPU. It is called SpeedswitchXP, and it is available
here.