I would try two things. First, stop and restart the SQL Server service (I am assuming that this is on your own box because I wouldn't recommend this otherwise). Second, if that doesn't work, reboot your pc.
I don't have Express at this location but I do at another location adn I will try this when I get there. I just tried this against my Developer Edition and it worked without a problem.
Author - Professional SQL Server 2005 XML