I could do with the motherboard model you have, It will give me an idea where the CPU frequency setting are (or someone else on the forum).
If you look yourself, It maybe where the FSB is set, it will be @ 133 change it to 100 and try it.
Intel Core2Duo 6400 @ 3.4Ghz
4GB Geil 6400 Ultra
3 x 500 Gb Samsung HD
EVGA 8800 Ultra OC