Hoping you already know Perian (http://perian.org/),
if you have problems watchin H264 AVI Videos on Quicktime (Mac OS X), the matter isn’t your video but Perian, there is a bug in Perian 1.1 corrected in the latest development version.
You can download the latest development version by follow these instructions,
Installing this version you will watch H264 AVI videos on Quicktime fine:
1. Go to System Preferences -> Perian -> Remove Perian.
2. Open Terminal (Applications -> Utilities -> Terminal)
3. In Terminal, type the following:
(Note: if it asks you to accept something, just temporary accept them.)
svn co http://svn.perian.org/trunk perian
4. Wait for everything to finish….
5. In Finder, use Go to Folder (shift+command+g), and enter: /perian/build/Deployment
6. Double-click on “Perian.prefPane” to install the new Perian.
7. In the new Perian preferences, go to the General tab and Install Perian.
8. Once you are done, you can remove the perian folder; to do this in Terminal, type:
rm -fr perian
9. Congratulations! You are done!