Why can't I change my desktop wallpaper/background in Windows 7?

Whenever I try to change it, I get a message that says "Access is Denied" It's my laptop--I've had it since black friday. It works great--but I want new wallpaper. My buddy was messing with my computer, trying to make Fabio my background (and thank god he was never able to do so) and I think he might have done something with the settings because now I can't change my wallpaper. Can somebody please help me out?