Recently I needed to configure Group Policy to lock user's screens after 30 minutes of inactivity, however it had to be done without invoking the screen saver. The article below provides instructions on how to do so by setting the screen saver executable name to:
rundll32 user32.dll,LockWorkStation
How to lock computers when idle by using GPO
http://www.bridgetonova.com/2007/11/how-to-lock-computers-when-idle-by.html
Mitch