QuickTime :: Why Is There Audio But No Video With Quicktime Player

I have had this problem quite a while now:When I open an .avi file it automatically opens using QuickTime. I'd like that because it's the best player for Apple.The problem is however that I can only hear the sound, there's no image/video.I use 10.7.2. (now updating to 10.7.3), but it already did this before I had OSX Lion installed.If I search for new updates it doesn't come up with an update for QuickTime.Any other player works (although VLC's the only one I tried so far). 

Info:
Mac OS X (10.5.8)