computer POSTS and then shuts down

I have a 12 year old home built computer running winXP sp3 and a 1+0 Adaptec software RAID. Replaced Power supply with PC Cooling unit a couple of years ago. Have replaced several of the Seagate Cheetah drives as the machine runs 24x7. The other day the computer started up and shut down in 3 sec before I can enter the BIOS and without any beeps. I took out and replaced the battery to reset the CMOS. I can't find any obvious shorts. I doubt the power supply has gone bad, but I do have a tester, just haven't done it yet. Other suggestions?

I had this problem a year ago, but at that time it took 10-12 sec to shut down. At that time I removed, cleaned, reappied thermal paste and reinstalled the CPU which took care of the problem. Thanks.
rwalker1Asked:
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Dr. KlahnPrincipal Software EngineerCommented:
I think you are on the right track.  Three or four seconds is about all the time it takes for a CPU to thermal shutdown if it is not making good contact with the heat sink.

If that turns out to be not the issue, disconnect all drives and see if the motherboard will boot without the drives.

Also inspect the four-pin connector which delivers power to the CPU.  These are prone to burn up that section of the motherboard if not connected solidly.  Look for burn marks on the white nylon connector.

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RaminTechnical AdvisorCommented:
It looks to be a multitude of different things, possibly the motherboard, CPU.
my first guess would be Power Supply, try to disconnect the power connector of optical drive and HD  and try again.
JohnBusiness Consultant (Owner)Commented:
Having had this happen, the remaining problem, and what happened to my XP computer in 2010 is that the motherboard collapsed. I had to recover data from the hard drive in a carrier.

XP is dead, buried and now fully rotted. Your 12 year old computer is probably starting to rot inside. Time to move on.
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RaminTechnical AdvisorCommented:
Test the power button on the front of your computer's case and also check connections.
Check for causes of electrical shorts , or defected power cable. replace cable, and retest.
dbruntonQuid, Me Anxius Sum?  Illegitimi non carborundum.Commented:
Suggestion only.

CPU fan gone?

Large desktop fan blowing into guts to keep air moving and see if that keeps it up and going.
Gary CaseRetiredCommented:
Assuming you want to keep using XP for whatever reason, I'd look very carefully at your motherboard for signs of deteriorated (leaking) capacitors.     Your problem sounds like one of 3 things:

(1)  Bad power supply
(2)  Capacitor failure
(3)  Failed CPU

For an older board like that, with a relatively new power supply, there's a very good chance the issue is capacitor failure.   If you see signs of leaking or swelling of the capacitors, that's fairly likely the problem.   If you see other components that are burnt, then don't waste your time (& money) replacing capacitors, as the board is likely beyond repair.     If you're comfortable desoldering components and replacing them, you can buy capacitor kits from several different sources;  or you can send the board in and have the capacitors professionally replaced -- probably the best-known site for that is www.badcaps.net

Of course you may also want to simply get a newer computer :-)    But if there's a good reason you want to stay with that setup, re-capping the motherboard will likely add years to its life.
nobusCommented:
what motherboard model is this?  check it for bad capacitors, bulging or leaking (see pic below)
measure the voltages of the power supply and BIOS battery - or replace the battery - it is a CR2032 and should measure 3 V at least

i recommend to power up with the minimum set up for troubleshooting, as described in my article, so disconnect ALL drives and cd's, and external devices to check if it powers down like that - then add devices till the problem is found :

http://www.experts-exchange.com/Hardware/Components/Motherboards/A_1945.html                  (Short-overview-of-how-to-troubleshoot-bad-hardware-when-a-pc-does-not-post)
JohnBusiness Consultant (Owner)Commented:
Question was properly answered;
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