Because different video files may be converted using different techniques with different codecs & different audio formats within them, there's actually many codecs you need. An AVI file can really be one of many formats because AVI is simply a containor file. What's inside could be mpeg 4 video or divx video, etc. & mp2/3 audio or mpeg4 audio, etc. There's many different variations & types. WMP is a program where you will need to install several codecs to play most types. I normally recommend VLC & MPlayer because they both have in-built codecs. In my experience, VLC can play more than DivX player can.