You can stream video other than .wmv if you use the program Transcode360 -http://www.runtime360.com/
Vista and Xbox360 are full of problems. I have tons of problems running audio and even Video using the 360 and Vista. However, if I take the same machine and install Media Center 2005 on it, everything runs fine. There is a problem with the Vista extender software and it does not work as well as it does with MCE 2005.
I am very frusturated with my Vista machine and just purchased it. That's why I pulled the drive out and through another one in and installed MCE2005 to find everything work fine. I put back in my Vista hard drive and regret it. I think I'm just going to format it and run with MCE2005 at least until Microsoft fixes all of Vista's problems.
I wrote a full article about it on my site in case you want further details:
http://www.technogab.com/?p=88