I'm running Windows 7 Ultimate. I found out how to disable the Guest user from shutting down at all, but then they can't shutdown even if Admin account is logged off.
Edit: To clarify a little, say I want to give my PC to someone, I usually click on Switch User and log in using the Guest account. That way, they can't access my account as it asks for a password. I want that in such a case, they shouldn't be able to shut down the PC as my admin account is still logged on.
However, if someone else boots up the PC themselves and uses the Guest account, then they should be able to shut it down too, as my Admin account hasn't been logged on.