Hard Drive

In Windows XP, there is an option to reboot on failure, but that could be a software failure or a hardware compatibility failure. Try booting into Safe Mode (F8 or F5 on most PC's when powering on). See if the PC boots to Windows. If it does, it's probably software or a specific device. If not, what message are you getting? Do you get Windows to boot at all? If so, it could be your power supply. Please reject my answer and post more info.

what os please?
if you have a bootable diskette you can rule out the hard drive by booting from the diskette (or cdrom). if it still reboots, something else is wrong, maybe bad power supply...? is it overheating maybe?

Yes and no. It depends on how bad the drive is. If the problem is bad sectors you may be able to recover by running scandisk, if however the mechanics of the hard drive are failing, then you are out of luck.

Does the computer ever stay up long enough to do anything?

Many things can cause continuous reboots: virus infection, bad hard drive, bad memory, bad cpu, over heating...

If you have a spare hard drive, you can eliminate that as a possiblity by putting the spare in the computer, installing an operating system on it, and seeing if it reboots...If no more reboots, then you have a bad hard drive.

If you have corrupt files, or electrical problems with the controller on the hard drive, it can cause a system to re-boot.

It all depends on where in the startup process the system re-boots.

If the system reboots in the midst of loading Windows, it is more apt to be a CPU or RAM issue.

Chas

I personally chased this particular demon over the last 2 days on a customers machine. Happened twice, both times starting on Tuesday of the past 2 weeks. Changed every pc. of hardware in the case including the MOBO and it continued.

Here's what I found when I was able to get in finally. On the MSN Messenger (I had previously used Shootthemessenger from www.grc.com, so I knew it wasn't Windows Messenger related, also had used DCOMbobulate from the same source, but ck'd. the DCOM ports and sure enough a DCOM port was stuck wide open and an RPC client was running on the port.) They had connected w/ an authorized user on the MSN Messenger that had a Blaster infected puter, then Blocked that user, but the port remained open, this puter had been patched against the buffer overflow problem concerning the Blaster Worm so it wasn't actually infected (Norton updated, Symantec Online, Stinger StandAlone scans showed no infection whatsever). But there was an RPC Server running and it was blaster infected on the other end. Was attempting infection of this machine (constant signal & traffic on the cable modem could be seen w/ nothing running).

As soon as I removed the offending user from the messenger I was back in business w/ very normal starts and no abrupt shutdowns...Had the customer call the user on the phone, he brought in his puter to me, cleaned off Blaster and a few other items (about 20 Viruses & Trojans...Lol). Sold him AV and updated...All is well.

Good Luck!