whenever I try and play a .avi file through windows media player or any other media player, i get a message saying 'error downloading codec'. what is a free decoder or whatever i need that will let me play these files?
I would recommend Xvid. Its free and you can get it from here http://www.xvidmovies.com/codec/ . It will decode Divx and just about everything else without any hassles. You could also go with Divx and they have a free version for watching avi files however I prefer Xvid over Divx myself.