If you've got DivX5.1.1. installed, please have a look at its settings on Tab "Quality" through [Start] "Programs"/"DivX"/"DivX codec"/"Decoder configuration Utility".
If "Support Generic Mpeg-4" is checked (selected) my XViD movies play using (i.a.) DivX-decoder.
(If this box is unchecked, same movies require XViD codec to be installed).