It's either dingo's theory or a short somewhere, which is very, very bad. You used motherboard spacers when assembling the rig, right?
Also, if you haven't tried clearing the CMOS, do so. Just keep in mind that it'll reset it, so you'll have to make any changes or overclocks as again. (*which you should do in the BIOS anyways. You probably know not to ever use software or "easy overclock" buttons, but I'll remind you anyways.)