If you don't want to use the possibility offered by BS.Player to allow it to install the necessary codecs internally (for use by BS.Player only), you could look for a (free) MPEG2 decoder (for DVD playback) e.g. here:
LIST OF CODECS (optional if BS.Player Codec Manager is used) - Official BS.Player forums http://bsplayer.com/en/bs.player/pro.../?faq=29#faq_2 codecs
And of course you can always resort to the option of using a codec-pack like e.g. K-lite,
making sure to deselect everything from it that you do not want/need for your purpose when you install it.
P.S.:
If you didn't allow codec-download by BS.Player at installation, but now would like to give it a try, you need not re-install BS.Player, but can run the application
codecmanager.exe (formerly
bspcodecdl.exe) which you can find in BS.Player's installation folder (XP-default: C:\Program Files\Webteh\BSPlayer[pro]\ )