Once again thank you for the follow ups.
Yes, I agree with bob3160 - it could well be any software that works with the mouse -
some background -
The first mouse was the standard HP USB wired that came with the desktop.
I must have been having problems with it - because I ordered a cheapo USB wired optical mouse from eBay at the end of Jan/2018 to replace it.
(I started using Avast Free Dec/16/2017)
Please note as posted earlier "
Windows device manager check for updated software says the mouse driver is up to date"
By mid April/2018 probably had double left-click (when single clicking) problems and installed CickFix -
https://www.clickfix.cf/and
https://github.com/CemraJC/clickfixThen I found the clicking problem was probably due to the cheapo mouse becoming slightly opened probably due to banging -
once I had pressed everything back together - things seemed to work fine - but I kept CickFix in place but reducing the delay time to only 21ms
chose that time from the pdf "A Guide to Debouncing" by Jack G. Ganssle
https://pubweb.eng.utah.edu/~cs5780/debouncing.pdfSo granted it may be the ClickFix software (and/or the cheapo mouse) the that's causing some incompatibility/interference with Avast Free
(and perhaps Kaspersky Free registration window)
But for now I am not going change anything while things seem to work,
as uninstalling and installing antivirus is not trivial.
Thank you all for your help