What rendering mode is selected under
(rightclick >) Options > Preferences > Video > [Video rendering]?
Could you try "Internal renderer overlay (default)" (allowing Aero to be switched off if applicable for your Vista-version) or alternatively VMR9?
Does BSPlayer recognize it as a video-file or mistake it as an audio-file (if you're using base skin, it does show this info). What info is shown under
(rightclick>) Options > Filters > Advanced (perhaps also click on the [+]-s).