AKICIF: Computer keeps rebooting

My computer is rebooting itself every few minutes, spontaneously. It seems to be a warm reboot -- it doesn't power down. It's not related to starting a specific piece of software, and it's not a mandated boot after an update, since there's no warning.

Any ideas what might be causing it? Or what I might try to stop it? This just started happening at (best I can tell) around 3 a.m. last night.

AKICIF: Computer keeps rebooting, part 2

Thanks for all the useful advice in replies to the previous post. This is an omnibus response.

1. I rebooted in safe mode mode, and it was fine for ten minutes, then it rebooted itself.

2. I left it at "Press Ctrl-Alt-Delete" screen that my illustrious company has set to come up before the signon screen for about half an hour. No spontaneous reboot during that time.

3. After grabbing the most critical material off the hard drive, I powered it down completely, then tried to power it up again. At this point I got a logon error early on in the process, something about how there had been an error the previous time it tried to boot and I should check something. At which point it would fail and repeat ad nauseam. Good thing I grabbed those files first.

So -- am I correct in thinking this probably rules out power supply and fan problems? And that maybe the motherboard or some such has gone wonky? It's sure acting like either an OS or hardware problem, not a software problem. Not a simple one, anyway.

In any case, the computer is going back to BT Counterpane, and the IT guy is going to try to get budget to get me a new desktop machine, since this one was the only spare he had lying around. I'm now back on the previous machine, the one that the new one was supposed to replace, because that one was acting wonky. All I had needed to do was move over my Treo files, and I was ready to ship the old computer back. Good thing I didn't yet, I guess.

I just love computers.