^^the Lemur nailed the primary culprit: heat. Secondary suspect is often: dust (which produces heat). Not every PC problem is software-related, which we often forget.

If, after you've cleaned, aired, and checked all your hardware, you still keep rebooting, be suspicious of your attached printers and/or scanners; they often are (by default) set to phone home for updates, and can cause XP to be confused about which task you want done first - thus causing a BSD/reboot.

Good luck.