Power down and restart

i would boot from the xp install cd, say, into the recovery console and just let it sit like that to see if reboots
yes, do turn off power saving stuff in Control Panel and BIOS for a test. You will see the power button configuration and/or Last Power State configuration there also. Sounds like when the power supply got changed the BIOS reset to defaults which probably left the pc off in event of power failure. hmmm. did you put this pc on a battery for a test?

One of my clients had practically the same problem, and it turned out to be some bad RAM.

Check out Memtest86 - A Stand-alone Memory Diagnostic

I've seen it before as RAM problem.

You can also use Microsoft Memory Diagnostic or the memtest86plus (www.memtest.org)

1. Check for virus after upfating your Mcafee or what ever you have to the latest release.
2. Replace its RAM & then SMPS and see.
3. Do you have UPS?

Ram True Problem, But Also Found That Your Motherboard Could Be Breaking Down, I Suggest Rebuilding Or Total Replacement! After 3 Years, Time To Retire, Upgrade To A NEW PC. You Can Continue Replacing Parts, But Gonna Get Expensive To Track Down The Problem. RETIRE & REPLACE My Suggestion.

Bee sure that the cpu coooling system is working properly - I had a situation where the fan had a piece of stuff lodged in the fan & it could not run. Cleared it and all was well.

Did you add any new software before it stated shutting down? We pushed Altiris out to various computers and they started shutting down as soon as you would sign on. We did a system restore for a date before our improper deployment and it fixed our problems.

First impression is bad RAM. If happens during memtest86 and you have more than one stick of RAM. Run test on one stick at a time as memtest86 uses very little memory but can crash if problem in initial memory registers.

Second Impression is bad mainboard and usually involves capacitors that filter power for CPU and/or memory. Inpect your MB closely in bright light and look at all caps to see if any of the little cans have bulging tops. If so that is the problem. Replacing the capacitors should restore you to full functionality -- but good luck finding what you need! Maybe best to buy new MB.

I had a similar problem recently with a machine about 3 years old.
Turned out that some of the capacitors on the motherboard needed replacing. One or two had "ucky stuff" oozing from their tops. Simple replacement and machine was back to normal.
I think it was a Soltek motherboard and the tech who came out to fix it suggested it was not the first of these motherboards where this had been a problem.
Maybe this could help.

John

Clean out your PC.

I just solved the identical problem on my PC-the CPU fan was clogged with dust.

It dropped the temp by 20+ degrees, no more shutdowns.

I agree with ajaycee. About 3 years ago motherboard manufacturers got a bad batch of cheap capacitors. I found 2 of my older PCs that were acting strange had leaking capacitors.