I suppose the video is shown correctly on your PC's (first) monitor (so that it isn't a codec-related problem)?
Unfortunately you do not give to much information on your hardware (graphics card).
Check BSPlayer's overlay-settings
(under Options > Preferences > Video > Video rendering)
If changed settings have no result also check video overlay settings under "Advanced"/"Additional preferences" of Geforce/Nvidia-graphics card's properties.
(For ATI card > advanced > select display > red button > then select overlay > cinema/theatre-overlay)