There is a utility called VideoMach that will let you save a file backwards. It also allows you to apply a variety of filters and is a handy little utility to have in your kit. I believe its free for non-commercial use.
Sorry, I assume people know to go to a search engine like Google.com to search for things. In this case, I should have given you a link because, unlike some utilities, VideoMach does have an official web site as Bryan (bansl) pointed out.
Once you load your video with a File->Open... then select File->Define Output... and on the dialog that pops up, go to the Video tab and check the box in the Frame Order panel labeled Save Backwards.