First you should find out whether it is vertically flipped or rotated 180 degrees. They are different. Vertically flipped is more likely and it's caused by codec incompatibility. This can happen when it is created with encoder A and played by decoder B. One codec interprets the specifications as scanning pixels from bottom up, the other interprets as top down. One of the solution is to try a different decoder, but the chance of success is quite low.

So, you actively flip it. ffdshow can play a wide range of video compression format. It can flip vertically with just a click in the menu. If you happen to have a compression format not supported by ffdshow (I can only think of RealVideo), or your video is not flipped but rotated, then use AviSynth. AviSynth can work for all cases.