How do I disable the Guest user from shutting down the PC while the Admin is logged in?

I'm running Windows 7 Ultimate. I found out how to disable the Guest user from shutting down at all via Local Security Policy.

This is the correct method. It is also robust in that it is unconditional.

If you wish to have your cake and eat it then the Guest account must use a powerful "agent" to perform the shutdown task. A scheduled task would be such an agent. It could work like so:

1. Create a scheduled task that runs Script1 to perform these tasks:
   - Check if the file C:\Shutdown\Shutdown.txt exists.
   - If it does not, exit from the script.
   - If it does, delete Shutdown.txt.
   - Check if there is a live Administrator session. If yes, exit from the script.
   - If there is no live Administrator session, shut down the machine, using shutdown.exe.
   The task must run once every 3 minutes under an admin account.
2. Create a desktop shortcut for the Guest user that invokes Script2 to perform these tasks:
   - Check if there is a live Administrator session.
   - If it does, create a pop-up to inform the user that the machine cannot be shut down.
   - If it does not, create the file C:\Shutdown\Shutdown.txt. The scheduled task will "see" this file and will shut down the machine.