Yesterday or the day before, my work computer freaked out and afterward, I was not able to change my background. Everything was greyed out. So, after searching my blog for the solution (cause I was sure I've talked about this before) but was unable to find it. So, I hit Google. I tried one solution, no dice.
Then I found http://www.thespykiller.co.uk/files/cleandesktop.exe you download it, and run it, and it will fix it so you can change your background.
You can read this forum which is where I found it.