Yep, as Dan already said, use Goldwave, fantastic program with many functions. Just open the avi file but make sure you have at least 2 gb's of space on your HDD as it will automatically create a temporary uncompressed wav file.
OR
you can use VirtualDub, just load the file and then 'File'--->'save WAV'. It will create a wav file which you can rip to an MP3 using basically any audio software.
Hope that helps m8