Thanks, Steve.
I seem, at the very very least, to have found a fix for this week's work. A step-by-step guide to help people getting just the error message I posted, here at:
https://helpx.adobe.com/creative-suite/kb/error-acrobat-8-professional-cannot.html
The second solution worked for me, as I was finally able (using the 8, installed by disc) to get a screen up to ask for my serial number. I put that in & voila!
Your thoughts make me nervous about this fix lasting but I'll take what I can for now.
Best,
Stacey