I haven't used Media Centre to stream to the 360, going to be giving that a go soon. But try googling TVersity. Excellent software for streaming to the 360. Im not sure if there are any file types Media Centre wont stream, but TVersity does everything.
Also you dont need to have MC installed to use TVersity. IMO its the best method for streaming, it's free for 360 and no modding required (ala Xbox). If you give it a go and get stuck setting it up give me a shout ill see if I can help. Although I couldnt get Media Centre to work off my laptop with the 360 at all. Im going to try it with Vista though, see if its better than TVersity.