Easiest Fix in the world =P I've been doing this for a long time but you needa use Windows Media Player not Center and this is what your gonna do.
Open Windows Media Player -
Then on the top you should see 3 little drop down menus...
Organize, Stream, Create Playlist
So click on the arrow of Stream and click "More Streaming Options"
A Window will pop up should recognize any devices hooked up to your network ( aka. computers, xbox 360, etc )
So find you xbox 360 and make sure it says "Allowed" next to it if not change it.
Then you needa to make sure you have your movies folder in your WMP, so...
Goto --> Organize -> Manage Libraries -> Videos
Click "Add" and browse to the folder containing your movies.
Now click on "Videos" in the sidebar to the left of the Windows Media Player and all your movies should appear in there.
Goto your Xbox 360 - Goto My Xbox -> My Videos
It will ask you to choose a source so choose your computer and then it should be good =P
NOW as far as MKV goes I have not yet found a way of getting them to work in Windows Media Player but I'll keep looking and let me know if you find anything!