Hard to tell
Here are a few things that you can check....
1) Make sure that the 'Media Type' tag on your avi files is set to 'Video'
2) Check the view scheme definitions for 'Audio' and 'Video'--eg. right click and choose 'Edit View Scheme'. Make sure that neither of the view schemes are explicitly saying to hide/show your avi files (could be by any number of characteristics, but the most basic is looking at 'Media Type').
3) If you are having trouble with child view schemes, you'll have to look at them as well...
My guess is that something on the Vista machine caused those files to import as 'Media Type'=Data, which means they won't show up in the Video view schemes.
As to why? Usually it's a flakey DirectShow filter.... Despite the problem with where the files are located, are you able to playback the files on the Vista machine?
brad