Back in September 2005, I bought myself:

Asus A8N-SLI Premium Motherboard
2x BFG GeForce 7800 gtx OC
3x 512mb Corsair 3200 DDRAM
AMD Athlon64 4200 x2
WD Raptor 74GB 10,000rpm SATA HD
Rosewill RP550 ATX 550W Power Supply

I put it all together and it worked fine. It worked for all these months without any problems that I knew of.. until yesterday. I'm in the middle of playing Battlefield 2, and all if the sudden my screen freezes. It's stuck on the same frame in BF2 and stays that way for five seconds. After the five seconds, my computer shuts down. I wait about ten seconds and try to boot it up. It starts back up for about 1 second, then shuts itself off again. If I keep trying to turn it back on, it will sometimes remain on. When that happens, I can hear the fans and the HD's spinning up like they normally would, but there's no POST screen nor any beeps. Just a blank screen on my monitor.

I checked all cord connections and made sure everything was still in place on the motherboard, but that didn't help. I tried a different power cord for the power supply, changed out the ram with ram I know works, tried an older HD that I know works, but nothing fixed the shutting off/no post screen problem.

It's obvious that this is a hardware issue, I just don't know what is broken or how exactly it broke. If the CPU OR motherboard is dead, would the computer shut itself off before the POST screen? I'd think if the motherboard was dead, it wouldn't turn on at all. Could it be the power supply shutting the computer off/preventing the POST screen because it's not functioning right?