Here is the Optional Media Update that will allow your Xbox 360 to playback MP4, Divx and XviD.
this will help some of you having problems with windows media center or watching some movies
How to use:
Open \Content\0000000000000000 and then copy the \FFFE07DF\00000002 folders inside there.
This will not run from a CD / DVD or work on a non jtag hacked console.
file added: Xbox 360 Optional Media Update (New in December 2011)
Optional-Media-Update-12-12-2011.zip