Firstly, this means that your system has problem with overlay, so the first thing to do is to upgrade your video drivers and directx ( but I don't know if DX9 is working well or not, so you may want to stay at DX8.1 ).
If it doesn't work, and if you use FFDShow as playback filter ( you can get it here :
http://prdownloads.sourceforge.net/f...3.exe?download, and I strongly recommend it for playing mpeg4 video ), you can, in it configuration panel - section miscellaneous, check "flip video". It's a quick hack, but it works.
If you don't want to use this filter, go to the preference panel of BSPlayer, in the 'video' tab, and try to change some settings ( check / uncheck overlay, change overlay mode, try checking 'use old rendering mode'... )
And finally, I almost forgot, one thing that can invert your video is having another window that use overlay ( typically : PowerDVD, DVD2AVI, another player ). In this case, BSPlayer can't create a second overlay, and it may so show the video flipped vertically.
I can't help you more precisely, because I don't have such problems on my system.