I've had this problem occasionally, and it is usually cleared up by deleting one of the xml configuration files. I believe that usually it is the library state file, but occasionally can be one of the others. (Since I'm at work, I don't know which directory they are in, but it is somewhere in the documents folder.) Open the files in your web browser (the default behaviour in my XP installation is to try and open it in IE). If there is an xml failure, you know the file is bad, so delete it.