Yes you can. You just need to get a unlocked cpu or unlock it. So you can use the multiplier to run at the cpu right speed.
Lets take a 1 Ghz 133 Mhz Athlon example:
7.5 x 133 Mhz = 1 Ghz
If you run it at 100 Mhz will be:
7.5 x 100 Mhz = 750 Mhz
Unlocked cpu changing the multiplier to 10x:
10 x 100 Mhz = 1 Ghz
Good luck!
Raf