Umm ... that has nothing to do with Vista, it has to do with codecs the Xbox360 supports. What you need to do to send other media types to your Xbox360 is get a transcoder. I can't remember what I used before I sold my XP box, but it let me play any kind of video that my computer could play, on my Xbox360. It transcoded the video and then sent that video the console.
Anyway, now that I read the rest of your post, yes ... Transcode360 is what I used, and it is the only option unless M$ releases a firmware update that has more codecs for other video types.
Just understand that all "streaming" is is "sending the file over the network" ... it sends the MP3, or MPEG, what have you, to the Xbox360, and the Xbox360 does the decoding. So changing the operating system that is sending the file will not do anything for the Xbox360's ability to decode video streams.
For example, I am using a Mac and I cannot send Quicktime videos to it without transcoding them, only WMV, etc, because the Xbox360 only supports those.
It's not that Windows XP won't let you send Divx files to it because Windows XP is incapable of do that, it's because the Xbox360 is incapable of doing that.
Do you understand what I'm saying?