How to ..
Avast is a free virus scanner. It will ask you to buy 'extra security' or a VPN but this isn't obligatory and isn't necessary.
Although we still don't have any actual viruses on the Mac there is the occasional nasty out there and you might also have PC viruses in files that you can inadvertently send to a Windows user...
It is a good idea to first run Malwarebytes free Adware removing software. Instructions here:
Then run Avast.
First download it from https://www.avast.com/en-us/mac and install it by opening the installer and following instructions:
It will ask you to install 3 extras during the install process so just un-tick them like this:
Continue to install and that's it! It will prompt you to Restart the computer.
From time to time you will see a green Notification on the top right of your screen as Avast has updated itself.
Or blue Notifications as it attempts to get you to buy the add-ons, but you can click the button saying Don't Ask Again.
You will also get a red Notification if it locates a dodgy file or email. If it advises you that it has been moved into the Virus Chest then you must open Avast from it's menu near the date and time:
Click Open Avast and you will see the Virus Chest in the list of offerings:
You will be able to select and delete the offending file/s.
First remove Adware with Malwarebytes then install Avast.