As far as I'm aware, XviD is an open source version of DivX, so if you're stuff's encoded in XviD, then you're already half way there
Xvid Source: http://downloads.xvid.org/downloads/...e-1.1.0.tar.gz
.avi is just a container type for video which can be formatted with a number of different codecs, including DivX, XviD etc. Incidentally, most distributions (exluding RedHat/Fedora and SUSE I think?) include mp3 support out of the box (Slackware certainly does).
Whether your distro (RedHat?) supports certain video and audio codecs out of the box is down to luck to a certain extent