First thing... Are rest of Aero features working or not?
If it is not...then this issue may caused by the video card or its device drivers. So first I suggest to go on vendor website and get new drivers.
Which ver of Windows 7? I think Starter doesn't support Themes & screensaver by default.
From start type 'Screensaver' and clcik on 'Change screensaver' to open the dialog to try.