Disclaimer: This is happening on my grandma's computer, I'm looking for a few tips before I go over there and try to fix it.
This is the thing: Last night there was a storm. For some reason, my grandma was pressing the reboot button and at that precise moment there was a blackout or something. After the lights came back, she turned the machine on. After the POST, Windows displayed the screen offering to boot into Safe Mode because the OS didn't shut down properly. She boots normally, and when Windows starts to load the desktop and shell, a dialog flashes for a split second and the PC reboots.
As I said, I'm trying to find possible causes for the problem. The most obvious thing seems to be some faulty hardware caused by the blackout and power coming back. How can I diagnose the problem? BTW, I don't think she has the Windows XP CD, so using the recovery console or whatever might not be possible.
EDIT: If anyone wants to know how it went, well, I wasn't succesful, mainly beacuse I thought my XP CD had the Repair option but for some reason it didn't. The error seemed to be some system file corruption (I think it was C:\WINDOWS\system32\config\Software). I tried running chkdsk from the recovery console, but whenever the checking got to around 70% it went back to 50% and went really slow from there, and I had other stuff to do. I told her to take the PC to a repair shop, where they'll have more appropriate recovery stuff. I'm accepting the first answer because it was the most complete.
Best Answer
The problem doesn't have to be hardware. Windows is entirely capable of destroying itself.
The only advice I can give is :
a. First examine carefully the Event Log.
b. Try to boot normally.
c. If you can't boot normally, try to go back to a system restore point previous to the problem.