did you ever get an answer to this?
I have just bought the AMBX starter kit in the hope that it will work with XBMC on Windows 7....
I've seen a video of it working but the guy who put it up seems to have forgotten to mention how!
nevermind.. the ambx forums mention that it supports xbmc,
New version AmbiBox 2.1.4 released.
In the new version added direct capture from XBMC