xp rebooting on shutdown command

i have an xp system that reboots when i issue the shutdown command.

this problem was not occurring prior to uninstalling symantec antivirus.

at this point i just need the quickest solution to prevent windows from rebooting. i.e., is there a registry setting that will completely disable windows ability to reboot?

any alternative suggestions (even if they are not quick) will be happily accepted too.