I have some avi clips that a friend emailed me. I try to drag one of the clips to the timeline and the voice is inserted, but the video will not insert into the video timeline.
If you know which type of file it is, you should be able to install a suitable codec... I had to do this when trying to use clips from my digital camera (motion jpeg).
Get yourself a copy of the free utility GSpot Codec Information Appliance and just open the AVI file with it and it will tell you what codec it uses and whether or not your system has the codec installed. It’s a great little utility for debugging video you download from the net.