This has happened to me in the past. I believe what you need to do is do a spotlight search for iTunes Screensaver. There should be at least one preference file that shows up. The file(s) will be named something along the lines of com.apple.screensaver.iTunes. In the spotlight search pane on the right side of the screen click the "Show All" Button. From this window drag all the preference files that show up and seem to be related to iTunes Screensaver into the trash. Then Start up System Preferences and see if that solves the problem.