Is it possible to use Cool N' Quiet on your motherboard ?
See section 3.6 in your user manual. There are no adjustments
to make if you use CNQ - it adjusts performance dynamically
for you, and will make the processor run cooler when it is
idle.
The current "CPU driver" is this file on the Asus download
site:
"ASUS Cool&Quiet Driver V1.2.2.2 for Windows XP/2003"