T'is a lack of AVI codecs, even more so if the file your trying to play is encoded with DivX. A google search for .avi codecs would be worthwhile, but aviod downloading the DivX player, as its a Spyware nightmare.
I cant comment on the latest version, but I've been using this for years and havent found anything it wont play yet.
Dunno.... Last time I downloaded DivX player was a few months back now and it sent Adaware nuts, since then I havent touched it and advise peeps to steer well clear, given the likes of the software at the other end of the link I posted, why chance it.