AVI files are a video format. AVI stands for Audio Video Interleave. AVI is the most common format for audio/video data on the PC. You will need a dedicated video player to open these files. See:
http://www.gnutellaforums.com/showth...threadid=27042
http://www.gnutellaforums.com/showth...threadid=26644
http://www.gnutellaforums.com/showth...threadid=26687
http://www.gnutellaforums.com/showth...threadid=26621
http://www.videolan.org/
http://mplayerosx.sourceforge.net/

I hope these help.