By default, XP is set to restart on errors. Go to Control Panel, open
System, go to the Advanced tab, click Settings under Startup and Recovery,
remove the check from "Automatically Restart" under System Failure.
This will cause the system to blue screen on errors instead of restart and
it might give a clue as to the source of the issue. Also, check the Event
log, go to Control Panel, open Administrative Tools, open Event Viewer, look
for errors that correspond to when the restarts take place, double click,
the information in the error message may give a clue as to the cause.
Finally, while at the XP desktop, place the XP CD in the drive, when the
setup screen appears, choose "Check System Compatibility," this may point to
problem devices or software on your system.