Bad PSU? Bad Mobo? hope not...

[mesole222]

I left my pc idle for about 3 hours last night, when I came back it was turned off. So I hit the power button and nothing happens. The most I can get is if I unplug the power cord to the PSU and replug it, the CPU fan twitches and does a 1/4 turn.

Yesterday I installed another Nvida 7900 GT CO VGA card to do SLI. I had everything working and tested it on some very demanding games (FEAR ran at 1600x1200 with all settings maxed with nothing less than 41 FPS). It worked perfectly fine until now, and nothing was changed. I'm thinking I may have blown the PSU, maybe? I'm hoping its the PSU because anything else would be very difficult to replace!

I have tried unplugging everything but the CPU, RAM, and Vid Card (just 1). It doesn't change anything. I've also tried a different wall outlet.

Everything in this system is pretty new and has not given me any trouble till now. Any ideas?

System Specs:
MSI K8N Diamond Plus mobo
AMD64 4000+ Processor
1 GIG Corsair Ram
Nvidia 7900 GT CO vid card in SLI
550W Antec PSU

**Update**
Took everything out of the box. I had 2 extra mounting screws, left over from a smaller mobo. Looks like these mounting screws were shorting out the board? I really hope this doesn't mean I've ruined the mobo.