Quit System Preferences. In the Finder, select Go ▹ Go to Folder... from the menu bar, copy the text on the line below into the box that opens, and press return:
A folder will open. Locate the file named "com.apple.screensaver.*.plist", where "*" stands for a long string of letters and numbers. Move the file to the Trash.
Launch System Preferences and recreate your Screensaver preferences.