mp4 files in Windows Media Center

First... according to Microsoft, the MP4 format is not one that is supported by Windows Media Center. Straight from the Horse's Mouth

This is actually not true, as Windows Media Player (which is used by Windows Media Center) will play MP4 files without an issue. So, it is just a matter of making Windows Media Center SEE and DISPLAY the files. How?

Hack the planet baby. Adding MP4 files to Movie Library I searched, and found this registry hack. It works. I have an XP machine running Media Center Edition. It too could not see all the MP4 files in my collection. I just downloaded the appropriate .reg file addition (x86 in my case since I'm XP MCE), ran it, and voila! the MP4 files are there in Windows Media Center. Didn't even need a restart.

Seriously.

If you want to play movies on TV, use XBMC, as someone already said, it's way better than Media Center. Also, if you want to stream your media files to XBox, try PS3 Media Server, despite it's name, it's very good DLNA server, supports most popular hardware, it can even stream(by encoding in background) files unsupported by target device. MKV on your console playing like a miracle ;)