prinzeugen42: Thanks for the replies - sadly didn't work - reran installation as administrator got same error message and then ran installed version as admin. Attached message below.
I think it's related to this:
https://www.gog.com/forum/general/gog_galaxy_wont_install But I couldn't really work out what there solution meant and I couldn't find the correct registry entries. I think it's to do with Windows 10 installing over the top of 7 - my wife was fooled by the red x to install thing a while ago...
Your issue is all about the permission. When you update to Windows 10 over the top of previous version of Windows it creates a default user with Standard Right, as far as I know.
Next tip might help you: there is Security tab in Properties option; look there and see if your user has Full control permission of that installer file.