How to disable XP shutdown (power off), but still allow restart & logoff

I really need to prevent users from shutting down their machines at the end of the day because that makes it impossible for me to connect to them remotely. (What could have been a 5 minute login fix turns into an unnecessary 1-hour drive across town...)

The only listed solutions I have found disable the entire shutdown option on the Start menu, or will also prevent restarting. That is not the right answer either, because the machine uptime will get out of control. I want users to be able to logoff or restart anytime they want. I just don't want them to see an option in the dropdown list that allows them to fully shutdown and power off the machine.

These sites have Windows XP Pro workstations on 2003 domains. I don't care whether the solution involves a group policy, local policy, or registry hack. Just as long as the only change is the loss of the power-off shutdown option.

Thanks to anyone who has the right answer for this one!