Xbox 360 is an amazing piece of hardware, especially with Windows Media Center. You can add all your media files on your PC to Windows Media Center and then enable it on your Xbox to play your movie, music, and photo collections on your TV. The only lacking piece to all this is support for MKV files, which are commonly used for HD video.
This is where PS3 Media Server comes into play. Yes, it's called PS3, because it was originally created to stream videos to the PS3, however it now fully supports XBOX 360. You just download it, install it, then add a few folders for it to host and then when you go to your xbox video folder a link will magically appear and you can select your movies and play them. The nice thing is it supports .MKV files. The only thing you can't do is seek forwards or backwards. So basically play and pause are all you can do. But it's enough to enjoy very high quality video on your TV through your xbox and PC.
To read more about PS3 Media server go here: http://ps3mediaserver.blogspot.com/
To download go here: http://code.google.com/p/ps3mediaserver/