Problem with Quicktime player playing .avi

[bigbadneil]

Why wont my Quicktime player play .avi and .mp4 video
Do I need to download anything else???

 

[Knoodles]

Perian

 

[toolbox]

FlipWMV for playing wmv files with quicktime.

 

[bigbadneil]

Perian

Installed it but still wont play .avi movies, do I need to do anything else??

 

[simsaladimbamba]

Use VideoSpec (I prefer that, though it hasn't been updated since 11/2009), MediaInfo Mac (10/2010) or MediaInfo (4/2011, there is a Mac OS X version to download, VideoSpec and MediaInfo Mac are using the same library as MediaInfo) to analyse an .avi and .mp4 file and attach the reports to your next post.
Have you tried VLC Player or Movist yet?