Previous versions of Windows XP Media Center Edition support joining a domain, but Windows XP Media Center Edition 2005 does NOT.
To share Media Center content with other locations in the home, you must use Media Center Extender devices, which require concurrent connections, only available by using Fast User Switching. This requirement precludes the ability to join a domain.
NOTE: If your previous Windows XP Media Center Edition was joined to a domain, the domain status and credentials will be retained when you upgrade to Windows XP Media Center Edition 2005. If you subsequently remove the computer from the domain, to enable Fast User Switching required by a Media Center Extender, you cannot rejoin the domain.
NOTE: You can still use domain resources if the shared resources do not require that your computer is joined to the domain.