Bad PSU? Bad Mobo? hope not...

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.

you have to try and hope for the best, although since it was already running when it shorted, it doesnt look good.

oh crap..... the other other bad news is that this mobo has been replaced before, and I don't think it can be replaced again under warranty.

just try it... if its broken, its broken. set it up on some anti-static surface outside of the case and hope for the best.

I wouldn't rule out a bad power supply at this point (I've had a few bad experiences with Antecs), but if it's just a matter of the case screws touching the board, while it's not ideal for the motherboard, it's probably fine. If the board actually shorted out a capacitator, the board would need replaced.

I'd take out the other video card and the RAM, remove the board from the case, blow it off, and put it on a piece of cardboard and try to get a basic POST.

thanks for the post nails and coop, i will try your suggestions!

i guess I should also mention that I got this power supply from my brother in law who had trouble with it. he said it wouldn't run with his SLI setup(not enough power), but it ran fine for me with just 1 card. I figured 550W is enough for SLI, but who knows, so hopefully it is the psu.

new update..

with just the cpu in the mobo it did the exact same thing. so tried another mobo with just cpu and it did a basic post. looks like I killed my beloved MSI K8N Diamond Plus. may it rest in peace.

my next problem is convincing the wife to let me buy a new mother board.

i just got back from the computer store. I guess i can get an RMA on the mobo.

sounds like it still might have been a power supply problem. I had a video card go bad ( the ram failed) earlier. the guy at the store said the PSU might have zapped the board or something. who knows. moral of the story, make sure you have a good power supply. i'm buying a new one.