I get a msg that says 'A required codec isn't available'. Help please.

Hi All

A friend downloaded for me Season 4 of the Sopranos onto a USB - she has a non Apple computer, the extension for each episode is -TNS.avi. When I go to watch an episode on my MacBook Pro, I get an error message saying, '...a required codec isn't available' I then followed the suggestions & downloaded Mac4Flip and the does not seem to help either. Any help MUCH appreciated.

Thanks in advance.