This is something that has always disappointed me about Quicktime. In theory you can view .avi and .mpg, but in practice, I have found a lot that can only be viewed in Windows. I've tried downloading additonal avi. and .mpg codecs for Quicktime but that hasn't helped much. Why Apple can't keep Quicktime up with all Windows standard codecs I don't know.

The only way round this on a Mac is to use Virtual PC (currently at version 5) and open the videos that you can't view with Quicktime in Windows. The drawback is that high quality mpegs will be play jerkily unless you have a G4 or fast G3 (as the emulation slows things down a lot).

If anyone has found codecs that allow them to play all standard Windows video formats on a Mac, I'd be glad to hear from them.