Why cant I choose a desktop background for my computer?

For some reason when i get to the area where you can choose your desktop background, all of the backgrounds are indented and cant be chosen. I can only choose a color for the background. I have a Windows XP computer, hopefully that helps. So what can be the problem and how can i get it back to where i can choose a desktop background. Thanks!|||You may have the resolution set to low. Make sure your graphics drivers are up to date and then check the display proerties.|||Um you might have a virus that is locking you out of part of your system so run a virus scan and see if you can choose a background after that.