Take a look at this. It seems to blow Windows media center out of the water - especially the network sharing bit - and it is free.
http://wiki.linuxmce.com/
http://wiki.linuxmce.com/index.php/Screenshots
http://video.google.com/videoplay?docid=-4422887272477313460&hl=en
http://wiki.linuxmce.com/
http://wiki.linuxmce.com/index.php/Screenshots
http://video.google.com/videoplay?docid=-4422887272477313460&hl=en