Both DivX-codec as well as XviD-codec can play either DivX and/or XviD encoded movie-files.
If both are installed, you must tell them what to do.
E.g. To arrange that DivX codec isn't used for XviD, you must uncheck/deselect "Support Generic Mpeg-4" on DivX-codec's "Quality Settings" (Default path: [Start] "Programs"/"DivX"/"DivX codec"/"Decoder configuration Utility").
Similarly to prevent XviD codec to play DivX-encoded movies, you should deselect "Decode all supported FourCCs (DivX, DX50, MP4V)" during XviD's installation (or thereafter).
(I personally mostly use Div-X codec with "Support Generic Mpeg-4" selected.)
With a video file loaded, BSPlayer will show under "Options"/"Filters"/(Advanced) what codecs/filters it uses.