Took me a bit of googling to do this (which really proves that I need to GET A DAMN LIFE):
I used an AviSynth script to do it, in particular this one:
In which “backwards.avi” is a copy of the video I wanted to reverse.
You can also use this script, but only if it’s an AVI file:
So what you do with these scripts? Once you have AviSynth installed, copy/paste into a text editor (changing filepath as necessary), and save it as whatever.avs. Then you drag and drop it into VirtualDub, which will execute the script and import it — backwards.
So then you test it. (I’ve noticed on about 50% of my reversed videos, it’s horribly choppy and unusable.) If it seems all right to you, set the compression and save it as an AVI. 🙂
If you don’t want to save it but just watch, just drag and drop it into a media player. The media player will think it’s an AVI file and play. (I’ve noticed VLC doesn’t work for this, possibly because its got its own built-in codecs and isn’t affected by AviSynth.)