If you like the theme that comes with Windows Media Center, you'll digg this Royale Blue theme created for Windows XP that emulates Media Center. 

I've been using this one for a pretty long time.. it's simple, and nice looking.  It doesn't change much, just some basic colors and shades, but something about it just perks up the basic windows theme.

Download the Royale Theme today and check it out.  It's about 670k zipped.