Is my power supply shot?

[Supermercado]

I've got a 400W generic power supply in my computer. I came home from school for about a week for Thanksgiving, leaving my computer at school over break. When I came back and tried to turn it on, it powered up for about 3 or 4 seconds, then shut off. I tried it again and it did the same thing, but only for a second or two. After that, pushing the power button had no effect at all. I unplugged it from my UPS for a few minutes and then tried again. Came right on and it stayed on until I came home yesterday.

When I got home last night and hooked my computer up and tried to turn it on, it did the exact same thing that it did after Thanksgiving, but this time it started off going for just one or two seconds and now when I try it (after leaving it unplugged for several hours), the power light comes on and goes right off.

I don't know all that much about hardware problems, but I think this is a power supply problem. Anyone have any other ideas for things that it could possibly be? There's no smell of anything burning or anything like that, it just doesn't stay on. Anything else that I should be checking before I spend the money on an Antec? Thanks.

Edit: After Thanksgiving, once it was running, it was business as usual. Nothing at all to indicate that there had been a problem.

 

[Supermercado]

Bump.

 

[Supermercado]

Desperation bump here...

I've tried a new power supply, getting rid of my generic unit with an Antec in the process. I've got a better power supply, but that wasn't the problem. It does the exact same thing with the Antec as it did with the generic. The only thing I can figure now is that it's the motherboard (unfortunately). Everything is still under warranty... since it looks like it's going to be the case, how is Abit with RMAs? Thanks.