Probably a malformed skin. Open a "Run..." box (you can do that from the Start menu), and type "regedit". Then navigate to "HKEY_CURRENT_USER\Software\Microsoft\MediaPlayer\Player\Skins" and you'll see a value that reads "CurrentSkin". Right click on it, and select "Modify". You'll see a path there that should point to your current skin, so change the path to point to another skin, like the default "Windows Classic.wmz" (if you use WMP11, that is).