Does the error in the event log correspond with the times that the PC reboots itself? What you could try is go into System Properties (right click My Computer (on Desktop) > Properties), then Advanced, and under Startup and Recovery, click the Settings button, then under System Failure, untick the box to disable 'Automatically restart'. XP tends to reboot at the slightest error, so while you obviously still have a problem, you may then get a more useful error message, instead of it automatically rebooting.