If you want to run Windows Media Center Extender, on your 360, you need XP Media Center, or Vista Home Premium (or Ultimate).
BUT, if you just wanna stream photos, music, and videos, you just need Windows Media Player 11, or Zune software, or other applications (xbox.com has support info for this).
Since Media Center Extender doesn't support DivX, but built in Xbox does, there's no point for Media Center.