Have you got all the correct codecs? I've found this is usualy the main cause of the avi files not being imported soo..
(try this first)
Make sure the movie will play (or if not go download ffshow this should sort out most things) then go in to movie maker. click tools/options. Go to the last tab and make sure that all of the codecs have a tick by them. Now try importing the file again.
AVIcodec is a program that will analyze the video file and tell you which codecs are needed, and which ones you are missing. Then, it will give you the link to the site where the codec is located, if you are in fact missing that particular one (click “web” button).
Hope this helps