Codec needed for .avi?

I have a .avi video file that will only play audio, says I need a new codec to watch it... can someone help and tell me what do I need?

Thanks.

ok. im not an expert on codecs either, but i think they are for decoding and encoding audio and video. so ull need to get a codec program, like here http://www.free-codecs.com/download/K_Lite_Codec_Pack.htm
its where i found out what the definition was. hope that helped.