"autoplay" in this context doesn't mean that BSPlayer will automatically start up if you insert a CD in your CD-player.
It means that a movie file starts playing automatically immediately when loaded in BSPlayer (without having to press [>] play button).
In order to "silence" Windows media player to the effect that it does not automatically startup if Windows has seen a media file (I think only XP does that), you should remove the file association with WMP and let BSPlayer grab it (apparently the type of file you're talking about is presently "associated" with WMP; in WMP look under "Tools" > "Options" > "File Types"; in BSplayer look under (rightclick) > Options > Preferences > General > File types ("Associated extensions"
(or in Dutch: Opties > Voorkeurinstellingen > Algemeen > Bestandtypes ("Verbonden extensies"))