PC suddenly shut down, will not power up - Help!

I need help diagnosing this problem. I have a home-built media center computer that just shut off suddenly the other night while I was watching a recorded movie. I would not power back on - no fans, lights, completely dead.

I took out the PS (Seasonic 400W), followed the instructions to test it (jumped the power on leads) and it worked. Tested all the pins and all showed proper voltage except the ATX pin 18 (-5v), it showed 0.

Luckly I had an unused PS (Antec 350W) and tried it in the PC. Same symptoms: no life at all. So I took that out and tested it the same way. Again all the pins showed good voltage including the -5v.

I also tried removing the tv tuners, unplugging the hard drives, removing the ram, even bypassing the power on button by shorting the power switch header, but the PS refuses to do anything (and I tried two different supplies!)

The only symptom leading up to this was two of my USB ports seemed to die a few days ago. Any relation to what I describe above?

The most important clue I think is I noticed some slight discoloration around pin 1 of the the PS ATX connector (3.3v). Looks like it overheated. I looked carefully at the MB and saw no physical issues with the caps or any other components. When I first opened the case after this happened, there was no smell and none of the components felt especially hot. I've looked at the internal temps with the bios at earlier times and never saw high temps.

Here is my system:

MSI K8N Neo2 Platinum
2x512 DDR333
AMD 3500+ Athlon with Zalman copper cooler (not overclocked)
DVD-RW
250MB IDI
300MB SATA
400MB SATA
Radeon 9800Pro AGP
ATI Wonder HDTV tuner
2x Hauppauge WinTV PVR 150
Seasonic Super 400 Silencer
Aphanix D4 case with two rear fans

I checked the PS requirements with the eXtreme PS calculator and got 305W recommended and have been running this configuration with a few changes/upgradeds for two years with only the usual(?) glitches one gets with PCs.

Can anyone give me some advice on how to diagnose the situation? I'd like to avoid just buying new PS, MB, CPU, etc until it is fixed.

Thanks,

Aermo

I'd concur with mpilch about the computer shop thing.

Alternatively, find a friend and use their computer as a "swap testbed" so you can see if the CPU and RAM still works.

RMA the mobo.

When I have dead parts and RMA them, they never ask any questions - I just get repaired or new stuff back. The manufacturer generally doesn't care what fried you're motherboard or psu, they just want you to leave them alone. Oh and that's not the first MSI K8 I've heard about frying. Send them back.