Media Center is like a shell extension on top of xp.
Or if you're asking another type of question. Media Center functionality, on top of xp, allows for a better multimedia experience...integrating tv tuning, pvr functionality, dvd, and more servicable music and photo playback.
You could use a pc with just xp for your entertainment center but media center allows you to see it in a visually more appealing way.
that should do it