A. Open VLC player. Then drag & drop the video file onto the VLC icon in the dock & see if VLC will play it.

B. If it says it's a quicktime player file but it's an avi, then that means QuickTime Player is the default program to open it. That means if you double-click on the file, QT player will attempt to open it. Problem is, QT player won't be able to play it unless you have the correct codecs. Best codecs for QT Player are 1. DivX, 2. XviD, 3. 3ivx.

For QT player make sure it has avi set in its settings. QuickTime Plug-ins & MIME settings (click on link)