Domain Policy to Enable/Disable APM.

I am a Systems Administrator and we just upgraded our Computer Labs to Windows XP. On a Windows 2000 Domain: How could I force or change the Screeen Power Schema?
I have tried GPO to foce a screen saver but I have not been able to find a Policy to DISABLE the monitors to TURN Off every 20 minutes if a user has NOT Logged on. I tried to change it in the Local Machine (change the profile from Desktop to ALWAYS ON)… but it does not work while the user is not logged on.
Any Ideas?

Thank you;