Really weird problem here. Basically, my computer was AOK when I was running xp 32bit, but i recently installed a new heatsink and formatted to 7/Vista 64bit. When I did format, and had my 6gigs of RAM running, the cpu usage would be crazy and my pc would be unusable. After exhausting every other option, I eventually tried running it with one stick of RAM; everything runs okay. All three sticks of RAM work independently. I have a XFX x58 motherboard, and 6 gigs of OCZ PC3
Sounds like a problem with either the memory controllers or one of the ramsticks, but I can't see how any of your actions would cause either problem...
Your Windows install could be bugged somehow, if you have an easy way to test it (for instance another computer containing a HDD with another Windows install you could try) then do that, otherwise i suggest you wait around here for better suggestions before trying to reinstall.
excuse me for the layman terms, but here goes: I have 6 RAM slots, one green, one black. my PC doesn't seem to want to load at all with the RAM in the green slots, but it will in the black. Unfortunately, if I have all 3 of them in I still get the CPU usage problem.