XBMC MythTV project over at sourceforge.net. I've recently played with it. It works great once you get it setup. It can play live or recorded television through XBMC This implies that you have MythTV configured and running on your network. http://sourceforge.net/projects/xbmcmythtv/
I think the Myth TV group have the guides pretty well organised. There is a lot more to this than just installing a few files on your xbox. http://www.mythtv.org/