If your Windows XP keeps spontaneously restarting itself this may be because it is set to do just that in the event of a system failure.

To alter this setting, go Start > Settings > Control Panel > System > Advanced > Startup + Recovery > Settings and uncheck "Automatically reboot" under "System Failure".

If you are changing this in order to get some error messages to help troubleshoot the problem, try checking your logs for clues first...
Control Panel > Admin Tools > Event Viewer