Hmmm... that's really strange. So GSpot can find the AC3Filter codec, but WMP can't???
Try uninstalling all your old codecs first (through add/remove programs) if you have any. Then reinstall AC3Filter.
If that doesn't work, try
VideoLAN. It has a bunch of its own built-in filters and doesn't rely on external codecs. I know that won't fix your MediaCar problem, but it will help with trouble shooting. If it doesn't work in VideoLAN either, there might be a problem with the AVI file.
Do you have this problem with ALL of your DivX-AC3 movies?