Hey guys, I wonder if you have any ideas. This is a spare computer that I was trying to get working. Turn it on, about 3-4 seconds later, shuts right back off. I tried changing ram sticks, hard drives, power supply, and reseated the processor. Do you think the mobo is fried? Any other ideas?

You could look up that computer's procedure for bios reset, like removing the battery, flipping some switch, or shorting something briefly.

Good idea! I used to know how to clear the CMOS (is that right?) but I can't remember.

Okay, so any suggestions if clearing CMOS doesn't help???

Anybody... PLEASE???

What is it? Specs etc? 

It's a Gigabyte GA-K8N51GMF-9 mobo, AMD socket 939. It's got an AMD64 2Ghz processor, 1Gb of DDR400 ram. It's probably 8 years old or so. When I updated my DAW computer, I gave this one to my daughter. A couple years ago the power supply croaked, so I bought a new one for it. Then after that, she would get random hard freezes. Then one day, it just wouldn't boot up at all. Turns on for about 3 seconds, then powers off. I swapped power supplies, 3 different sets of ram sticks, reseated the processor, swapped hard drives, took the battery out, used a jumper to clear the CMOS, disconnected the power button after it turned on. There was no video card installed, so I thought maybe the onboard graphics took a dump. So I put a video card in it, no change. Can you think of anything I haven't tried? At this point I'm just thinking the mobo got fried. I'm out of ideas.

If it's been inactive, there's probably a lot of dust inside that could be causing something to over heat.  Take it to a well ventilated area and try cleaning it out with a can of compress air. 
 
Also check the power supply.  You can find a test for around $10-$30 at a computer store. 
 
If it's not one of those, you may have a problem with the motherboard or the CPU. 

I've had this happen to my mothers Dell a few weeks ago. She was on vacation and left it unplugged for a while. When she returned that is what was going on with hers. Turned out that for some reason the bios reset and ACHI was chosen instead of ATA or IDE for the HD. Changing it back got it sorted for her.

Appreciate the help guys. This computer was in regular use up until this happened. I tried cleaning everything. I was actually surprised to see it was very clean already. I tried a different PS, no luck. Tried the faulty computer's PS on a different machine and it worked well. Last night I made the decision to throw it away. I'll change the title. Thanks again for your efforts to help.