I have been helping someone who has several computers in his large home office. I have shared all the computers so they can access files and folders on the other ones. I have one laptop which has XP Media Center and although I set an exception in Windows Firewall to allow file sharing, and even tried turning firewall off completely, it will not allow it. All the computers have the same username when they start up, including this laptop, but this one keeps challenging for name/pwd. If I turn on Guesst account, then instead of the challenge I get a Windows warning, access is denied.
Check whether the username/password is the same as the one you set on XP Media Center, by right you should be able to access the shared folder on XP Media Center with same username/password.