turtle007 & Mew Gull,
To Apple Discussions!
Open the file with QuickTime Player.
Open the Movie Info window (Command-i).
Note the name of the video codec used in the file.
Do a Google.com search for codec name +Mac and download one appropriate for your OS and computer.
Quit and relaunch QuickTime after installing the new codec.
Try the file again.
Thanks to QuickTimeKirk for the above info.
If the above does not work, then use
VLC Media Player or
Perian (AVI and FLV support)