Windows XP Media Center Edition 2005 is exactly like Windows XP Professional Service Pack 2 except it doesn't allow domain login and there's a couple of other things. It allows IIS if you insert an xp (with sp2 slipstreamed) cd then go add/remove programs,
There are a couple of things different though. Media Center has an improved start menu (the colours don't burn your eyes, they look nice :O). Also it's arguable that Media Center has had optimisations to explorer and other stuff that makes it run a lot faster.
Note: There's no drivers for the XP MCE remote control so you need Windows XP Media Center Edition installed for drivers. No workarounds IIRC.