From released notes of the latest 6.4 version: Known issues: DivX Codec does not yet support Microsoft Windows Vista. And there is no newer beta atm.
wtf, i swear i had divx installed and playing back divx encoded media without a hitch.. encoding however i never tried.
I just booted into my Vista partition ( Vista Ultimate X64 Edition Build 5728 ), I am using an older version of Divx 6.2 but I am able to watch Divx videos, I updated to 6.4 but I don't do any encoding ( I don't make any home made pron ) so I don't know.
i tried encoding on 32bit vista, seemed to work without complaining and played back fine... so i'm a bit confused.