Virtualdub can read DivX codec format. Provided you have a full DivX compatible codec installed anyway, and not the crappy Video for Windows thing. Getting XviD (another free codec, which I prefer over DivX) should be enough to use DivX too.
Whoever ripped it was probably being a dumbass and naming the files ".divx" because it's a buzzword amongst the average internet user for video, a bit like mp3 is for audio - you'll find they're normal .avi files most of the time and can just safely rename them.