system
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thanks for the prompt reply, ditto for my email to support!! 
I googled for the norton tool and ran it. I had previously run the control panel uninstall when installing Avast. (Maybe when installing Avast you could warn us about the need to uninstall using the tool versus control panel. It might have saved me months of aggravation. I didn’t suspect the dueling viruses could be my problem.)
So far, the pc is running better - only one day later. If it still crashes frequently, I’ll go back and run the three programs in you post. When I ran the removal tool, my pc kept sending Microsoft error reports until I shut it down. Those did not continue after re-booting.
Thanks again for your prompt help!