Grab Perian which will grab all the necessary codecs.
Apple recommend Perian: http://www.apple.com/downloads/macosx/video/perian.html
Thanks again Guys,
I was on my Powerbook G4 with an avi with great picture but no sound.....loaded in Perian...and it worked like magic. I knew the video was OK cos it was already working on my IMAC.
Perian rocks.....and Quicktime is still the best player in town. I think the DIVx player is cool but Quicktime with Perian plays everything......