This fix is windows specific. AVIs are now evaluated by the pre-FFMPEG code first, failing that only by FFMPEG. Adding the 200 frame testfile from the bugreport http://projects.blender.org/tracker/?func=detail&aid=6707&group_id=9&atid=125 now goes fine with Add>Movie. Add>Movie+Audio will complain about the fail first, but it will be added anyway.