AFAIK, that's a problem of the Xvid decoder but I'm not 100% sure.
Remember that XVid is still under heavy development and that there were some buggy builds out, which even concerned the decoder part of the codec, that part that you use when merely
watching XVid video.
Try using latest Xvid codec which includes latest decoder. Find it here:
http://www.roeder.goe.net/~koepi/.
Maybe I'm totally wrong, so please check out
http://forum.doom9.org at XVid forum for precise information.