Unfortunately there isn't a converter. Although it would be ideal. Part of the problem, is it requires the xdk to view the xmv file, and so a converter would have MS code in it and would be illegal to distribute.
It is a simple matter of using xmvdecoder in the sdk to open a video and audio stream, then capturing the video and audio to an avi file. I have looked into it, but lack some of the programing skills to pull it off.
Also, I would be hesitant to release the file because of the legal issues, but you never know, someone out will probably do it soon.