customize video thumbnails

[Lucy]

I need your help. I have been collecting tv ads for
thirty years. Wow have I got a lot of ads. Many on vhs
which I have been able to digitize.

Now I have two hard drives full of .avi files in various
folders named autos, beverages etc.

I want to select which image from the ad is the thumbnail
that shows in my windows explorer when I go to a
particular folder.

The thumbnail that I choose will be the image that best
represents the ad not just the default first frame of
each video.

Thank you

Lucy