The Media Center is [just an application for Windows. It allows you to watch TV, listen to radio, view slideshows, movies, etc. (with the right hardware). The computer runs XP Pro, it just comes with that additional application. Your choice whether or not to use it, if not, you're just running regular old Windows.
I think the common version I've seen preinstalled has been with XP Home not XP Pro.You normally have a choice of Home or Media Center for no extra cost but XP Pro is extra.Media Center is designed to be used as an entertainment center/controller and for use with a remote control.It's hardly worth cluttering a new PC with unless you are getting it to use as an entertainment center.It also has been the source of some troubles/problems/conflicts.