Codecs

I have windows movie maker version 6.1 i think but all i know is that its not the latest version and when ever i put in an AVI file it says "cannot be imported because the codec required to play the file is not installed on your computer. If you have already tried to download and install the codec, close and restart Windows Movie Maker, and then try to import the file again." but i have got the codec installed im really sure but i don't know how to check this. Also i have tried restarting the thing and it still doesn't work. also i don't know where to get the codec from and what other file types would work. I am in desperate need of help so thank you if you answer back! :D