Have you tried converting it to other file formats aside from AVI before converting them to AMV?
I had the same problem once, well a little different cause I got an error message when trying to convert AVI (even if the program supports the file format) and what I did was I converted the AVI to mpeg format first and then tried to convert it to AMV and it was successful.. A little time consuming though but really worths a try! ^_^
Well I just hope this link will help you to acquire video converting programs... that is if you want to try it out!
Just click
HERE