[BSPeter]

Is your PC's OS a secret? Sorry, but "bsplayer pro 2.1", that's indeed quite an old version 2 you're talking about (July 2006?), which I don't have installed on my PC any more. So maybe update? Otherwise:

Originally Posted by BSPeter

adicoto refers to the option "Enable drawing subtitles under image with EVR renderer"
under (rightclick >) Options > Preferences > Subtitles [Subtitle properties].
This - of course - only applies if you're using EVR (Enhanced Video Renderer) (default under Vista and Win7).

To make this reply more general in case you're not using EVR and also for the possible benefit of other users, I add some other options concerning subtitles which may be of interest:

(In the same window as the one mentioned above)
Is perhaps the box "Draw subtitles to Overlay surface" selected?
If so, deselect/uncheck it.
(If you have "Draw subtitles to overlay" enabled, subtitles can only be drawn on overlay = on the video picture, and not below.)

In connection with "overlay": what is selected as "Rendering mode" under
(rightclick >) Options > Preferences > Video > [Video rendering]?

Are you using new subtitle engine or old?
(See the box "Use new subtitle parser/renderer" under
(rightclick >) Options > Preferences > Subtitles [Subtitle properties].)

Finally there always still is the "Expand picture to"-option
(rightclick >) Options > Preferences > Video > [Video rendering]
(Option at bottom/right)
See this post (and the thread it is in) for some clarifications.

P.S.:
Always, I do mean always, mention version and build-number of your BS.Player!
Also it may very well be relevant to know which OS and videocard you're using.

Originally Posted by BSPeter

With respect to the option "Extend picture to" (under Preferences > Video > [Video Rendering]) I already indicated that I think the 576 line option has to do with 576 line interlace PAL and the 480 line option with the 480 line interlace NTSC display.
Further, as long as you're really "extending" the picture (which depends on the original (real) ratio of the movie vs the screenratio of 4:3), it works.
E.g. if in full screen mode the background is shown as black bars at top and bottom of a video.
By "extending" you can then add similar black bars to the picture itself so that the black bars shown are in fact no longer shown as 'visible' part of the background, but they have become part of the movie-picture itself shown in front of the background.