Codecs, codecs, codecs. Do a Google search for "Windows Media codecs avi" or something like that. You are bound to find something or other.
Otherwise, get a different player that will play most of the files (google up "VLC" or "mplayer").

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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-