Don't forget about the GlobalSearchService in windows services.

I have the same problem on a VM attached to a network. All services with Sage in the name have been stopped. Global search is not available for Sage 300 perpetual so that's not it. I removed all PrintBoss printers. I deleted Semaphor.bin. I am still getting this message and am stumped. Sage 300 2020. The machine had a workstation setup on it and I am trying to change to a C drive setup because of a problem with FR. 

Edit HKEY_LOCAL_MACHINE\SOFTWARE\WOW6432Node\ACCPAC International, Inc.\ACCPAC\Configuration\Programs and change it to C:\Sage\Sage300, then install.

For the first time in all of my installs/re-installs I had to edit the registry as suggested by Team Equation.  In the past, when I have run into this error during install, it was just a matter of stopping one of the Sage related services but, in this case, it was on a terminal server that previously had a workstation setup (now installing full program files as required).  The uninstall of the w/s doesn't seem to clear the registry.

Thought I would chime in so folks know this is a good place to look for why it 'thinks' Sage is running or in use.