As described in http://blog.davidesp.com/archives/389, I was unable to successfully open .avs files in VirtualDub (1.9.11). I had AviSynth 2.5.8 installed under Windows 7. I tried uninstalling and reinstalling AviSynth, but this time stepped back to AviSynth version 2.5.7. It worked – VirtualDub (1.9.11) can now open .avs files and display their images. Not sure if it was just the clean reinstall or the step-back to 2.5.7.