By default, XP is set to restart when there's a system error. It's possible
that this is what's causing the behavior. Causes for the error vary and can
be software or driver related. It will take some effort and detective work
on your part to uncover the cause on your system. To help you with this
process, see the troubleshooting guides for XP shutdown problems at this
site: http://www.aumha.org/win5/a/shtdwnxp.htm