Unlike previous versions of Windows, XP has native support for processor performance control. The operating system has built-in support for Intel SpeedStep and AMD PowerNow! Thus, the applets previously required for use with Windows ME, 98, et cetera are no longer necessary.
Navigate to Start > Control Panel > Power Options and set the power scheme to Home/Office Desk or Always On.
Home/Office Desk > None (AC) > Adaptive (Battery)
Portable/Laptop > Adaptive (AC) > Adaptive (Battery)
Presentation > Adaptive (AC) > Degrade (Battery)
Always On > None (AC) > None (Battery)
Minimal Power Management (AC) > Adaptive > Adaptive (Battery)
Max Battery > Adaptive (AC) > Degrade (Battery)
Source:
http://www.dslreports.com/faq/5635
Adaptive = Performance state chosen based on CPU demand
Degrade = Starts at lowest performance state, then uses linear performance reduction (stop clock throttling) as battery discharges
None = Always runs at highest performance state