How to keep guest account from shutting down Windows XP Pro

I remotely manage (using VNC) a computer for a friend’s business. The problem is that the receptionist continues to shut down the PC even though it has been repeatedly stressed that it needs to stay on. This, of course, means that if I need to maintain the computer during off hours I must drive to the business, which defeats the purpose of remote management. How do I either:

a. Disable the “Shut Down” option on the Guest account

-or-

b. Set the PC to automatically reboot after a shutdown?

It seems as though it should be easy to do either one, but for the life of me, I can’t figure out how to do it.

The PC is running XP Pro.

Thank you so much for any assistance anyone can provide!