it is XP pro. it just has media center add to it. it comes on 2 discs instead of 1. with about 26 min left on install, it tell you to put the 2nd disc in for about 3-5 minutes. then you put the first disc (xp pro) back in.
the only feature that you lose is the ability to join a domain (they disable the part, you can still network and have remote access control), but I have a hack to reenable that part