Ok,
.avi is just a container

Inside that container will be some video & some audio
it's the audio & video that needs the codecs

Download a program called G-Spot
http://www.headbands.com/gspot/v26x/GSpot260RC1.zip

It's a program that will look into any .avi file and tell you what the audio & video inside are encoded with, and whether you can decode them properly

-------------

Open up Gspot



File & open
Open your AVI file

Gspot will show you what's going on inside



As you can see my example has
DivX 5 video & MP3 Audio
and both can be decoded ( "Codec(s) are installed" )

You can test if everythings ok, by rendering the file
Under "proposed Codec Solutions and tests" at the bottom,
click the "1" button under "MS A/V"
this will tell you how Gspot proposes to decode the file



pressing the "2" under "MS A/V" will play the video




Any problems, (ie. can't decode something)
let me know (screenshots if poss.)
and we can get the right codecs for you

Hope this helps

- Tony