I haven't modified anything in the windows system folders. First I only had a problem with the computer restarting before I logged in (it came to the welcome screen but rebooted before I could log in). Then I reinstalled XP. I had the same problem, and it also started to reboot after I had used the computer for a while. Now, it doesn't reboot before you log in, but it reboots after instead...
The time between reboots is usually a couple of hours, and it has been like that about all the time I have had the problem. I don't think it is happening sooner and sooner.
Reboot jsut after log in happens maybe one out of two or three times when I start the computer.
I agree that it sounds like it could be a overheating issue, but the temperatures seem to be normal...
Anyway, I reinstalled XP a second time tonight, so we'll see what happens. If it didn't help I'll get back to you with another post.