Someone posted a solution in another thread on the same subject in the Sophos help forum for the Free Tools, so I'm posting it here in case someone else has the same problem. It was a Microsoft snafu. Bookmarked the Fix-it just in case I ever need it again in the future.
I was having same issue. If you still have the latest version installed you could try downloading Microsoft Fix-it and use this to uninstall Sophos. A tip I got from Sophos.
After running the uninstall I suggest rebooting Computer.
Next try running the Sophos install again, and finally when it's finished install, reboot the PC (don't try to open Sophos before rebooting).
With luck on re-boot you will find it now works. Not sure why this worked for me but it's worth a try - I've been trying to solve problem for a few days, but it's the first time I tried the reboot after installation.