Good news! I got it installed. Here is what I did:
The error was occurring during installation at 38% of unpacking data files. I found an article at this url:
https://support.symantec.com/en_US/article.TECH23733.html
The only part applicable to my situation was removal of all files in my %temp% folder.
I also took the additional step of deleting the folder C:\Program Files (x86)\Altiris
I then rebooted the system.
Installation now worked.
As I recall, I had cancelled my first attempt at installation. I can’t help but wonder if that is what got my %temp% folder to get in a weird state.
I hope this will help others.