Create another user and test with that.
When did it work properly, is it recent enough for System Restore to be worth a try? See if System Restore will get you back to a restore point before your problem with Windows, OR try the System File Checker.
How to repair the operating system and how to restore the operating system configuration to an earlier point in time in Windows Vista (or 7)
Perhaps there is a recent update or installation that started the problem.
Does the Slideshow Gadget work if you launch that?
Are there any updated Video Card drivers available, if not does rolling back to the previous driver solve anything?
See how many unsigned drivers you have running on your machine. This article will guide you on the instructions but all you have to do is type sigverif in Start Run.
How to identify unsigned drivers using sigverif utility in Windows Vista
Try running ChkDsk to check your drive for errors. Right-click your Drive icon/ Properties/ Tools/ Error Checking. Try it first by not checking either box (this will run it in a Read-only mode) to see if it flags any hard drive or file problems. If it does, restart it by ticking both boxes, and rerun it to allow it to attempt to fix any found problems.