Sometimes the player itself can reject the codecs, which I find really strange. It happens to me regularly with the Windows Media Player, and has happened on occasion with WinAmp. Be sure to try playing the movie with VLC, the, in my opinion, best video player around:

http://www.videolan.org

"Stephen Wolfram is the creator of Mathematica and is widely regarded as the most important innovator in scientific and technical computing today." - Stephen Wolfram