848P-A(V2.0) MoBo not booting

My cousin has a computer that has been built from multiple components that he had laying around, or was able to afford at the time. The system has been working for the last several years(maybe 3-4).

The other day after WotLK was released for WoW, he went to install it, and realized he didn't have a DVD drive in. I installed a Sony drive for him, and he was up and running. 2 days later, and the computer won't start.

I took the computer, and have been looking into why it won't boot. I have tried a new PSU, and that didn't do it. it will usually spin the CPU fan about half a rotaion, and not really startup. I then stripped everything off of the MoBo, and connected just the ATX1 connector. It booted.

Then I started connecting other things to the MoBo... RAM worked, sound card, worked, Graphics card, worked, and Ethernet... worked.

So I was confused, then I connected the ATX12V 4-pin connector. Stopped working. I removed the mobo from the case, so it isn't shorting out against the case. I have tried it many many times now, and every time, with the ATX12V cable connected it won't boot, and without it connected, it powers up fine. But the cable has been connected the whole time. Also, I don't have it hooked up to a monitor right now, but I don't think it is actually starting, or getting past post.

My main questions are, anyone have any ideas as to what the problem is, is it a bad mobo, CPU? I don't have access to other components without buying them, so I am looking for your best guesses.

Also, I believe that the ATX12v connector is the one that powers the P4 it's self.

Gimme your best answers please. Thanks a lot.

-tehrob