Can't change desktop background

In the Display Properties window, on the Desktop tab, the Background list has
been disabled - the list has a solid grey background, nothing is highlighted
and the sliders won't work. It is the same on my daughter's profile, but my
son's works fine. I think SpySheriff 'protected' my wallpaper, but when I
emailed them I got no reply. I have since uninstalled Spy Sheriff, but the
changes are still in place!
My daughter got round the problem by right clicking on a picture, but even
this didn't work for me.