How can I find which codec an AVI file needs? A method that does not require downloading a program would be preferred (such as looking at the file in notepad).
Open in notepad, do a search for 'vids' - the FOURCC is right after that. Eg you'll find something like:
Then google for 'fourcc WMV1' in this example.
This is a very simple process and can be done using the GSpot Codec Information Tool. Just download the program and install it. Now go into your Start Menu and open the program.
Note: Do not download anything labeled as a "Codec Pack" or from the links at the top of the FourCC website. This will help you stay out of trouble when playing your files.
VLC has support for Divx and 3ivx built in for all platforms.