Thanks! The new ISA plug-in works great!
Warning for anyone else attempting the same fix:
If you download the new AvPxyFtr.dll file with the Avast ISA filter running, Avast will corrupt its own download. If you then install this corrupted file, ISA Server will refuse to restart. If you attempt to download AvPxyFtr.dll once again, this time with Avast disabled, you'll find that ISA Server has cached the corrupted file and ISA Server will still refuse to restart.
I suppose it's possible to purge the ISA Server cache, but I don't know how offhand. It's also possible that the initial download would not have been corrupted if Avast had been disabled in the first place.
Finally, I downloaded the new AvPxyFtr.dll file on my home computer, which is not running Avast. I then transferred this file to the server via floppy disk. This procedure seems to have worked perfectly.
As a test, I deleted one of the local WSUS downloaded files and then synchronized my server with Microsoft WSUS. The replacement file downloaded successfully. Last night, Microsoft issued another Critical Update. It was downloaded without problem. Thus, it seems that Avast's new AvPxyFtr.dll file is not corrupting downloads as it did previously.
Ken