I looked for good instructions that already existed for how to do this, but all the ones I found had too much noise. Microsoft’s own pages were pitching you to upgrade from their free products to their paid ones. Please don’t be fooled by that. At least as of this writing, they still offer free antivirus and firewall.
In the search box on the bottom of your screen, search for “Windows Security”.
Select the Windows Security app.
Then go to “Virus & threat protection”, and check that it is running. It may have a yellow triangle, and “Set up OneDrive” button. If it does, click “dismiss”. It should either show “Real-time protection” or “Virus & threat protection”, with a green checkmark. If you see any red, that probably means something is turned off. Turn anything that is red on. If it shows yellow with a dismiss button, you can safely ignore it.
Do the same for the “Firewall & network protection” section.
Please reply here if you have any troubles or questions.