It could be the codec the .avi was encoded with. Do you happen to know what its codec is?
On a side note, winamp doesnt have the best video support in the world, so it could just be a bug in the software. I am sure you know of the other great video players out there (VLC, MPC)
Not to get off topic, but VLC doesn't need codecs for almost everything.. thats the beauty of the application. What was it your were trying to playback?
aprox is right, winamp is sucky for any type of video. if i recall correctly there was an mplayer plugin for winamp, that'll handle almost anything, similar to VLC. winamp doesn't even utilize directshow it seems, since wmp plays the file just fine