Always try the simplest fixes first. I'm glad I did before attempting the mind-numbing convoluted solutions people love to come up with on forums like these. Among all the responses I found on the Internet, the simplest was this:
Uninstall anti-virus, restart the computer, and reinstall the anti-virus. Success.
Just so you know, I had Windows 10 on my laptop for a few months and then all of a sudden the left-click didn't work on my Start button and on other icons in the system tray. By hovering over the Start button using my mouse with one hand, and then left-clicking using the laptop touchpad with my other hand, I was able to open the Control Panel and connect to the Internet. Then, I went to
PandaSecurity.com, scrolled to the bottom of their home page, and in the footer, clicked "
Free Antivirus 2016," followed the links to the download, and downloaded the totally free program to my hard drive.
I then went back to the Control Panel and
uninstalled the free copy of Panda Antivirus
that I already had on my laptop (to which I have a large-screen monitor, keyboard, and mouse hooked up). I also made sure that no other antivirus programs were still on my machine, like my expired copy of Norton 360.
When I restarted my computer, my mouse worked just fine. I hopped back on the Internet and reinstalled Panda Antivirus from my newly downloaded copy, and that was a success, too, and my mouse still worked fine. I ran the Panda program, selecting to scan only the critical areas, and in a few minutes it scanned 73,000 files, and deleted a cookie. Mouse still worked fine. To make sure, I shut down my computer. Waited a minute. Started it back up. Everything fine.
Again, always try the simplest fixes first.
