This is a bit off-topic for here, but I have been tackling this problem for a while now and wanted to let people know how to solve this problem if they encounter it.
When Media Center 2005 came out, certain features were crippled in order to support MC extender devices. Among these were the networking options. By default, a new MC machine cannot join a domain except during setup. This caused much consternation in the community and resulted in a lot of thinking that this meant MC was based on the Home Edition, not the Pro version. This isn’t the case (entirely)… MC was built on the Professional version, but the networking components were relegated to more of a Home edition status. One casualty was that it was no longer possible to save usernames and passwords for network resources (which is what happens on XP Home based machines).
However, the “hack” that will let you join a MCE 2005 domain will also let you re-enable the saving of passwords. This shouldn’t affect extenders as long as you don’t actually join a domain, but I haven’t tested that feature yet.