The latest Xvid codec from
http://www.koepi.org with the latest Divx codec from
http://www.divx.com together with the AC3Filter from
http://ac3filter.sourceforge.net/ will allow playback & encoding of 99+% of all AVI's "out there".
Why screw up your system and install one of those Codec Paks, they do more harm than good and the codecs within are almost never the latest anyway.