I can't run .avi files with OS 10.8

Why I don't see .avi files with OS 10.8? Usually I could run .avi files in my iMAC o Mac Book Pro but after I turned to OS 10.7 or 10.8 it was not possible anymore. Some older avi files use to run in Power Point presentations but not the new ones. Anytime that I try appear a sign that refer to there is no valid codec to run the clip.

Thank you.