Assman2,
At the moment the 360 will only accept WMV files streamed from either Windows MCE or XP running media player 11. As well as a NAS to stream MP3 I use an old XP SP2 machine running media player 11 to stream movies the the 360.
It's dead easy to set up. Just launch media player, go to Tools | options and select the library tab. Configure the sharing and the folders to monitor and away you go. If you go to configure sharing and the xbox 360 isn't shown attempt to connect to the PC from the xbox and when it fails go back to the PC and allow the xbox 360 in the configure sharing area.
Enjoy...
PS Rumour has it that a later update this year might include the ability to stream DIVX/XVID to the 360 as well in an attempt to make it like XBMC on the old xbox's, fingers crossed.