I think you can open the .avi file in Quicktime, then Show Movie Inspector, in the Window menu, and it will give you the codec in the Format info. Not sure if it works the same on Windows though. If that doesn't work, then the next step is to determine:
Where did the video come from? If you exported it from your video editing software, the codec should be something you select when you export. What software in particular you are using will affect the exact process though. If you let us know what software you are using, we can probably be more specific.
If it came straight off of your camera, the codec it uses should be in the documentation for the camera.
If it came from a third-party source, you might check with them as to the codec.
Also, its probably worth mentioning, in case you didn't go to the support area first, that there is a knowledgebase article
that deals with this exact issue.