In Lion follow these steps:
1. Quit all programs then click on an empty part of the desktop until the only the 'Finder' is in the menu bar.
2. Hold down the 'option/alt' key while click on the menu item 'Go'. Then while still holding down the option/alt key select 'Library'.
3. When Library opens scroll to the folder for "Preferences" and open it.
4. In Preferences find the file: com.apple.systempreferences.plist and move that file to the desktop.
5. Launch System Preferences->Desktop & Screensaver and try again to see if it is fixed. If this doesn't fix it close System Preferences, follow the steps back to the Library->Preferences and put that file back. If it work you can just trash that file and smile.
Mac Pro Dual 2.8 Quad (2nd gen), 14G Ram, Two DVD-RW Drives, OS X 10.9.1
2006 Mac Book Pro 2.16 (first Gen) OS X 10.7.5
2TB Time Capsule, 2 TB
Black, iPad (3rd Gen) 32G Black