I've been looking into getting BeebMC to run on the 64 bit version of Vista. It runs fine, but plays no music.
The reason is that Media Center runs as a 64 bit process, and there is no 64 bit Real Audio plugin for Internet Explorer, just the 32 bit version.
You can run a 32 bit instance of IE and use that to listen to the BBC, but when I host IE in BeebMC, it is running in a 64 bit context, so it can't then load Real Audio to play the BBC streams.
If/when the BBC starts streaming in a format that Windows Media Player can play natively then this problem will go away (and you'll be able to use BeebMC on extenders.