Try this. It may be what your looking for.
It's in Windows 7 and Vista.
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Control panel, Power Options/Battery icon, Change plan settings, Change advanced power settings, Processor power management, Minimum processor state (I have mine at 50% and at idle the system will idle around 1.6 ghz, setting to less is really for laptops as it takes longer for the processor to speed up on demand, it's quick at 50%), Maximum processor state - Set to 100% if not already there.
Doing this should have you system idle, use less power and be cooler.
When you need it it instantly bumps up sometimes in the middle and if you need it it goes full speed and speed step and c state set in bios still continue to add the extra mile to it.