This problem appears to have been solved by removing malware from a flash drive attached to the computer and from the desktop.
Last night I ran MalwareBytes Anti Malware with a full scan and it found four malware items (see the attached screen grab). MalwareBytes said to reboot the computer and when I did Aero was back and screen graphics were back to normal.
I have not shut down and rebooted again, but I am pretty sure the OK graphics
will persist and the problem is solved.