I would say the program files are missing, wouldn't you think?

Intriguing...since I signed in and used the application yesterday!

Yesterday was 24 hours ago, a lot could have happened since then. Like someone could have deleted or moved program files - I see this happen a lot.

Click Help, System information and you will see a list of Applications. If the Name is not spelled out (like Accounts Receivable)  and only shows a 2 digit code (like AR) then the program files have been delete/moved.

It was an Avast problem. I included the relevant folders in Avast's Global Exclusion list in Settings (all in Avast). Then ran the Sage install files in Repair mode, ran the upgrades and successfully got in.  

Ha... anti virus strikes again. Wish I had a dollar each time this happened.