Yesterday I bought and installed "Exterminate It" and "Registry Reviver" plus a driver fixer. I ran the first two -- found a hijacker and repaired a hotkey, and fixed over 600 registry errors. My computer ran better for half an hour and then the troubles returned. Ran Exterminate It and found nothing. Ran Registry Reviver and found 63 errors, all file path problems and empty hot keys. Computer ran better for a little while, but then crashed. I was having difficulty shutting down so I did the "Switch User" test and when I used the little red button was told another user might lose data if I shut down. There still should be no other user. This was repeated several tmes till I finally gave up. File path errors had disappeared and empty hot keys dropped to 46.
I sent a report of the computer's state to the folks who own Exterminate It. We'll see what happens next.