windows xp restarting for no reason

I have windows xp, it came with the pc when I purchased it in december. Since a couple of days I have been having troubles though. My computer restarts for no apparent reason at all. I called compaq support and the tech person was so nice and helped me although he shouldn't ( my software warranty expired). I did the restore to an earlier date and it was fine for maybe an hour until it did it again. Because I didn't know what else to do I factory restored it with the recovery CD, again without any change. I found a webpage that told me xp does that because of system errors, and sometimes for no reason, and showed me a way how to turn that option off. Again it did not help. I don't know what to do anymore.
Can anybody help me at all, please?
Thanks

 

You will need to rule out overheating. Remove the cover and see if the fan/fans is/are working properly. Blow off any dust that may have accumulated. As a test, direct a desk or floor fan so it blasts air on the internal boards to see if that prevents the reboot.

I had a person with this same problem and it turned out that he was kicking the power cord strip under his desk and causing a short interruption of power which made his computer reboot.