P5NSLI will not post

I built a computer using a P5NSLI and I think a Pentium D 805 processor. I had the processor overclocked a little, but not to the extreme (I can't remember exactly how much.) At some point the computer started rebooting itself, and eventually it got to where it would not power on at all. So first I replaced the power supply, which did nothing. Specifically, when I try to power the computer on, the fans (including the power supply fan) spin for a second and then the whole thing turns back off. If I try to power it on again the fans come on and stay on, but I never get a POST. So then I figured it was probably a problem with the motherboard, so I bought a new one, also a P5NSLI. So I got it home and tried to boot it up, and it also will not POST. It never posted, not even once. I then figured wtf maybe I have 2 bad power supplies, so I tried a third that I knew for sure was good, to no avail. Any help would be appreciated, because I am stuck.

bios was not updated at all, on either board. The new motherboard has never been booted up not even a single time. I cleared the cmos on the old motherboard but not on the new one. When it restarted it would not go to the blue screen, it was as if the power was going out. I was using Windows XP Home edition.