I’ve never really had this issue since first using  Vista Media Center, but thought I would pass along this little tidbit of information since I’ve seen it on a few different forums lately.

The issue involves the Video Library in Media Center and the inability to remove video locations that you have added (or even default locations).  If you find yourself in this predicament, you may need to refresh your Windows Media Player database.  I don’t want to repeat a good set of directions, so head on over to the Microsoft Knowledge Base and check out KB925718.