Leucius: Sometimes you need to do an OS downgrade/upgrade, and Witcher 3 may be installed on your secondary drive, like me. So it
was installed correctly, however when the OS was changed, it no longer shows in Control Panel. Maybe I'm a one off, but I doubt it.
Hickory: If that is the case, then you should have qualified your remark as such. Changing OS with software installed is not a good idea, anyway. The sensible (read 'correct') course is always to back up data, uninstall, change OS, re-install.
My bad :) I know it's not the wisest move, but I only have GOG games, Steam games, and a couple MMO's installed, and GOG I know I can uninstall via Galaxy, Steam I just have to run once to get the correct registry entries, and MMO's I've learned you just run the installer and it puts the correct registry entries in.
For some reason The Witcher 3 didn't do that, and never has in all the OS switching I've done, but all my other GOG games do.
I'll try to remember to be more specific when I try to contribute to tech support threads.