If you are using Windows XP, then www.divx.com has a codec and player. Dont use their converter since its trash. Their divx codec dosent work 100% on Vista.
To conver DVDs to divx, use AOA DVD Ripper. I tried many DVD2Divx programs and its the best by far.
XViD works fine on Vista and unless you want to play divx files on your TV/DVD Player, its a good choice.
I wish the Dvix people who come out with a version that is stable under Vista soon.