Are you sure that it isn't rebooting due to an error?? Have you got xp set to reboot automatically? If so it might be hitting an error as it shuts down.
You should also check in the bios that it's not restarting through a setting for wake on lan etc.
There is a registry adjustment that can stop the reboot problem if it's a registry setting error...
http://www.kellys-korner-xp.com/regs_edits/poweroff.vbs from
http://www.kellys-korner-xp.com/xp_tweaks.htm is one.
if that doesn't work then we may have to dig a little deeper