It depends on what you mean by multimedia pc
For multimedia production:
ubuntustudio
For Multimedia/home entertainments:
mythbuntu or
mythtv
I'm guessing that you meant home entertainments, watching movies/tv, listening to music etc, in which case, I really don't think you can do much better than mythbuntu if you still require your pc for normal desktop use, or mythtv if you intend for the pc to be dedicated solely as a media player.
If it is multimedia production you're interested in, then I am a fan of ubuntustudio, but there are other linuxmedia distros, although most would still argue in favour of XP.
