The AVI file is a basically a wrapper that can contain a multitude of different formats of video and audio. If you don't have the correct codec installed on your PC, you will not be able to play any AVI that makes use of that format but it is often not simple to figure out what codec you need. To help get round this problem, Richard Thomas has written a helpful application that you can use to determine which codec a given AVI file needs. You can download the program from:
Run it from a DOS prompt using the syntax "qaviinfo <avi_filename>" and it will provide information about the contents of the AVI. The output will look something like:
File Info: # of streams : 2 Size : 300x224 File Type : AVI Default File Handler Stream Info: Type Codec Rate Scale Length vids IV32 12 1 58 auds 11289 512 106
To determine the video codec, look for the line starting "vids" in the "Stream Info:" section. The FOURCC of the codec is given immediately to the right of "vids". Knowing this, you can cross reference the FOURCC on the Compressed Formats/Codecs page of this site. If you are lucky, you may even find a link to download the software you need to play the AVI.
If you don't want to download and run Richard's app, you can also try this simple method to determine the FOURCC. If you open the AVI file in Windows Wordpad (in Start/Programs/Accessories) you should see something along the lines of "vidsXXXX" somewhere in the top few lines of garbage displayed. In this case, the XXXX is the FOURCC that the AVI file needs to decode its video.