Original hardware:
CPU: Athlon XP 2200+
MB: ASRock K7VM3
PSU: ATX 375W
Memory: 2x 512MB DDR 266
OS: Windows XP MCE SP2

After working flawlessly for a very long time, I started experiencing computer shutdowns everytime I plugged in USB devices that used the USB port as the power source (iPod Touch and external HDD). I thought this was caused by a defective PSU, which I tried to change for a ATX 500W. The computer failed to start so I then thought the motherboard was causing the problem. Further tests with my original PSU allowed me to pin-point that either the motherboards IDE ports or both my HDD (tried individually) are defective as the computer freezes on the post when either of the HDD are plugged in any of the IDE ports. The computer even freezes that way when a HDD is connected to the IDE without power (disconnected molex connector). I opted for a defective motherboard.

That's where it gets tricky.

I ordered a used ASUS A7V333 rev.2.00 motherboard on eBay, certifited as being tested and working properly.

I plugged everything in and whenever I pushed the power button, the computer starts for 2 seconds and shutdown, before the post. Nothing is displayed on the screen as the hardware doesn't even have time to start properly. Therefore, the problem is not software-related.

I then started isolate the cause of the problem.

I tried removing all the hardware except the original CPU (with properly connected fan) and the original PSU. Same pattern occured.

With only the CPU and the PSU connected to the motherboard (A7V333), I cleared the motherboard's memory. Still, the same shutdown after 2 seconds occured.

I took care of verifying that the motherboard is properly set to JumperFree mode to make sure it's not bad CPU frequency settings that cause the problem.

My parents have the exact same computer hardware (and their computer works properly) so I tried using their CPU and PSU on the A7V333 motherboard and still, the computer shuts down 2 seconds after I start it.

So, with only the CPU, PSU connected to the motherboard, tested with 2 different PSUs and 2 different CPUs, the computer continues to shut itself down after 2 seconds.

I'm running out of options. Could be a defective motherboard, but I'd be pretty unlucky to stumble upon 2 defective motherboards in a row.

Anyone has another idea before I throw everything out the window and order a new computer?

Thanks!

Have you checked to be sure your power supply is highenough to support it? It may not be highenough. I had that problem before so I went out and bought a power supply with more watts and It worked.

Motherboard should not touch case...check for it.

hello
You give allot of info , thank you
the fact that you have a new motherboard and a new power supply tells me you have enough new stuff to go to the agent who sold you the stuff let them set it up , that should be included in the cost in any way , i am a computer technician an d whenever i buy anything i let their techies do the work mostly because then they can fix their own problems and because the last 2 power supplies, cpu's , ram and motherboards I bought both motherboards was dead one power supply was dead and the ata cable out the bag was faulty
Must be a motherboard, cpu or ram
Ram I don’t think so, motherboard, it sure sound like, cpu who knows
Inspect your cpu slot with a magnifying glass to see if their is not maybe a short, some dust particle? Verify each component if you can if you cant , TAKE YOUR MB TO BE TESTED they should have triplicate of everything to be able to verify what’s happening.
Hope it helps
barry

As RJCO said the motherboard should not touch the case try removing the motherbard from the case and running it without the case. It sounds like maybe ou need some thermal solution or maybe it's a faulty CPU and you need to replace it. The thing is these things are difficult to diagnose without matching parts that work to test with.

Thanks for your input guys.

Will try that with a power supply from the office.

Replacement motherboard (ASUS A7V333) is a used one bought on eBay. Can't have it verified or installed by the store.

Will try that tonight to make sure, but isn't the motherboard supposed to be screwed to the case at some point? I don't understand how I could screw it in and use metal piece that surrounds the connectors at the back of the case without the motherboard ever touching the case.

As I said, I tried with the CPU from my parent's computer which is still working properly in their computer. So faulty CPU seems ruled out to me.

I have access to a perfectly working PSU and CPU which are the exact same models as the one from the faulty computer.

Also, it can't be the RAM as I'm testing without RAM installed.

You have no RAM in the system?

No. Tested without the RAM. But correct me if I'm wrong but that shouldn't prevent the computer from staying powered on. A computer without RAM will beep at the post to report the RAM problem, not shut down after 2 seconds.

In this case this is most likely something to do with a faulty motherboard...

I know this seems like a daft suggestion (I am just guessing know as it deffinetly seems to be a MOBO issue yet you have replaced it) but I had an old AMD system with a sticky power button. Mine would do the same unless you pulled the power button out .

Other than that, it seems you may have damaged the motherboard when fitting it. It is difficult to explain but just a surge of static can not neccesarily destroy but deffinetly damage the part.

This is a very parculiar issue. I mean usually that means either a PSU, the CPU or the motherboard its self. But you have mix and matched the three with seperate parts.

And yes you are right on the beeps. Without any RAM installed your motherboard should continously warn you that no RAM is present.
That is what lead me to beleive that the motherboard maybe faulty but it's probably not had chance to give you the warning.

One question, do you get anything on the screen during the 2 seconds that the system is booting?

A sticky power switch (or reset switch) is definitely a possibility. It's the front cover of the computer where the switch panel is that sticks, not necessarily the switch itself. Many times I've had to oil the switch to stop it from sticking. Also check that the CPU fan and PSU fan spin up to full speed quickly.