Man, I can't understand why Quicktime will play some .mpg but not all. Sometimes it just says that it doesn't "understand" the file... ok....
Well, I don't know if this will work for OS9 but try getting a hold on DivX doctor, an Avi-to-Mov app which will change .avis to the much larger .movs (duh).

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iMac G4 OSX 10.3.9
RAM 256MB
LW 4.10.5 Basic
ADSL anything from 3 to 8Mbps/around 1024kbps

"Raise your can of Beer on high
And seal your fate forever
Our best years have passed us by
The Golden Age Of Leather"
-Blue Öyster Cult-