You can use Gspot to determine what codec is needed.
http://www.free-codecs.com/download/GSpot.htm
I use GomPlayer. It's a nice free player that has most codecs packaged with it.
It is possible that the file is corrupt. But first try gspot or gomplayer or vlc.
vlc is a nice free player
http://www.videolan.org/vlc/
GomPlayer
http://www.gomplayer.com/main.html
Good luck!