Thank you! I suspected that might fix the problem, but thought it prudent to ask first. The uninstall was fast (via Windows Settings menu). With the link below, Piriform emailed me the registration key for my Pro version. I downloaded the new software and installed it, all in a few minutes - the latest version 5.25.5902. CCleaner now runs perfectly for me. If only all of life's problems could be solved this easily. Thanks again for your help, and thanks to Piriform for making the process simple and effective.
http://www.piriform.com/docs