what i did was i got a burned DIVX cd-r of a movie.
i'm guessing xvid works too,i mean i've ripped a few dvds with xvid i think.well,just to make it small enough for my ipod anyway...

if you try play it on the xbox,it SHOULD ask you to download a codec for mp4 if you're connected to xbox live.mind you,this also lets your xbox play DivX and XviD files.
i don't know about RMVB though...
i'm not sure about this,but i think xbox should have native support for MPEG and WMV files.well,WMV anyway,i'm not sure about MPEG too much.

also,i really don't know about windows media center,i'm using windows media connect,to make things a lot more convenient.i can play my music during games,especially when the in game music sucks(ahem burnout...ahem avril lavigne....)