It also depends on how STRICT the policy is. Does it re-apply itself during the day, does it LOCK registry permissions on the keys, etc...

The location is here for the screen saver.
HKEY_CURRENT_USER\Software\Policies\Microsoft\Windows\Control Panel\Desktop

By renaming the KEY Desktop or removing it will at least temporarily disable the policy. However a GPO can also be set to prevent a normal user from modifying this key as well. All depends on how tough the Admin is on the subject.