CS5 goes up to 6.7.1 and your current version is 6.0.0 You can't install 9.1.1, that is for CS6.
Photoshop should also be 12.0.4 and not 12.0
Download the AAM update Adobe - Adobe Application Manager : For Windows : Adobe Application Manager
and run Help > Updates
OK... sorry to sound a bit dumb here, I didn't realize the 9.1.1 version wouldn't work for CS5, but I'm still stumped on your reply. "run Help > Updates" ... where? I dowloaded the link you attached, [and I thank you for your support...] but I'm not sure where this "Help>updates" pull down lives. In CS5? Because the Help menu there doesn't have an "updates" tab. Or in the AAM... which shows two files called "Packages" and "resources" and a "Setup", which I setup. Then it also downloaded the Creative Cloud APPS page to download programs. But I don't see any "help>updates" point to go to.
Run the setup to install the AAM update, then open Photoshop CS5, go to the Help menu > Updates and you can download and install all updates for CS5.
thank you thank you thank you! I had been avoiding updating from Windows 7 to 10 because I'd heard all sorts of nightmares about compatibility with applications, but I eventually had to go ahead and do it and I think this might have contributed to my problem. But.. it's all good now. You are awesome! Many thanks to you.