It's not that it cannot handle them. It's just that it lacks the codecs as it is to play them (that, or the vids you dlded are corrupt).
I have come across some like these which either VLC will play or, if they are too corrupt, Mplayer will do it.
The codecs can or can't be found, either from Apple or from third parties but I think that VLC is better because I'm too lazy to go codec-hunting on the net. Of course, the interface of VLC isn't nearly as fast as QT's (or Mplayer's for that matter) but every player offers some features that the other doesn't.

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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

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