boot - PC shut downs automatically after 10-20 second. No POST screen, no beeps

I have this not-so-old computer that's not being used for a year or so.

Specs:

• Motherboard: ASUS PN5-E SLI


• CPU: Intel Core2Duo E4300


• RAM:2x2GB SuperTalent DDR2-800


• VGA: Zogis GeForce 7950GT


• PSU: Vitsuba San-55-S 550w


• HD: No hardrives yet

When I power on the computer, everything seem to start, but right away the whole system shuts down.

I've removed and changed the RAM sticks, take out the VGA, everything I could think of.

So what could it be causing this? The PSU? The motherboard is dead? The CPU?
Any help to isolate the problem will be useful.

Thanks

PS: Please don't close the question, this could be helpful to anybody having a similar problem, even with different hardware.

UPDATE

• I've removed the old thermal paste and apply a brand new one.


• I also cleaned every dust using a high pressure gas dust remover.


• Checked for bad capacitors, all of them seem ok.


• Opened the PSU, removed big giant dust balls, cleaned with high pressure dust remover.

Still the same problem, but now it stays powered on for almost 20 seconds maybe. But no POST screen, no beeps at all, nothing.

So I suspect it's a motherboard or PSU failure. Unfortunately I don't have an energy tester to test the PSU...

Don't know what else to try. I don't have another 775-motherboard to test the CPU, RAM and VGA with it.

Answer

Off of the top of my head, I'd guess a bad power supply. You could check it with a power supply tester, but sometimes they don't fail unless they are under load.

As EKW is pointing out, you could be looking at some sort of thermal shutdown. I guess I could believe a year in the right conditions dried out whatever thermal compound is installed and left you with a gap. How long does it keep running after you hit the power? You might visually inspect the capacitors on your motherboard to see if maybe you've got a bad capacitor, as suggested, too.

During this year in which it was not used, how was it stored? Was it unplugged? If not, it might have been damaged by a power surge. Temperature? Humidity?

Even with the info you've provided, there's not a lot to go on, and the best you're probably going to get are guesses. Assume everything is faulty, and if possible, swap in known good components where you can get your hands on them. You could also try your components in other systems. Hopefully you've only got one bad component and you'll be able to isolate it via a process of elimination as you prove components good or replace them with known good ones.

I've done this dance many times. Remember to kill the power (at the plug) before taking stuff apart or you might let the magic smoke out. Good Luck!