Has one or more of the panes in System Preferences had its icon change to include a light switch without affecting the pane's functionality? This issue can be resolved by doing the following:

1. Quit System Preferences if it is open.
2. In the Finder, click on the folder in the Finder's sidebar with the house icon, or for systems running Mac OS X 10.2 or earlier, on the item in the Finder's toolbar with this icon.
   
3. Open the Library folder in the frontmost window, followed by the folder named Caches.
4. Throw the item named com.apple.preferencepanes away, and then reopen System Preferences if it was open previously.