On the evening of 13 April (Pacific time), I tried downloading again from the Download button at
http://www.classicshell.net/. I got something different, but it was still bad. The downloaded file was only 59,495 bytes when past versions were over 6 MB.
I then downloaded from the Mediafire mirror at
http://www.mediafire.com/download/iwcm9 ... _4_2_1.exe. This was 6,590,656 bytes, approximately the size I expected. It installed well.
Today, I again downloaded the installer via the Download button at
http://www.classicshell.net/. This time, I got a file that has the same SHA1 hash as the file from Mediafire.
CONCLUSION: Something was temporarily wrong with the download server at
http://www.classicshell.net/.