Have you tried the divx component for MacOS X, divx.jamby.net ?
This can decode MS MPEG-4 v3 streams (avi). It also includes a
avi2mov converter which makes the problem with avi-files go away !
(It also makes the files streamable, avi isnīt)
I donīt know if thereīs a version for MacOS 9, but there you can
use the DivXPlayer-WMP thing ...
Hope that you can find a way to play the movies ! I think the Jamby
solution works OK, even if itīs not Altivec enhanced. No encoding support,
(ie encode to DivX, but you can convert to other format from DivX)
(Iīm using a pbg4-550/256/20/DVD)