May have a hardware fault, first suspect is an extremely good chance your mother board or power supply is suffering from bad cap syndrome.
In a nutshell the electrolytic caps on the system board have dried out, changed value and are on the verge of popping. This causes instability of the system board.
XP by default is set to reboot on a default in the system properties tab under startup and recovery.
Uncheck Automatically Restart to stop the automatic reboot under the system failure heading (this may be helpful for troubleshooting but Windows may still do things it shouldn't since the fault is most likely not fixable unless you change the bad caps or try a different power supply).
The caps can be changed but you need to get Low ESR types and you are not going to find those at your corner Radio Shack..
I am thinking your PC has reached End of Life.