Cybersecurity Basics:
Viruses, Spyware, and Other Malware

Avoid Viruses, Spyware, and Other Malware

Make sure that any anti-virus software is enabled, and that it is constantly updated.

The Windows operating system itself now includes Windows Defender, leave it have turned on. At that point most personal users are fine and you don't need to buy antivirus software for your personal systems. That sounds like heresy, but read this article for the details on how Windows Defender is adequate, and added antivirus software is at best an unnecessary expense, and at worst it can make security worse. If you want to add something, the free version of Malwarebytes is probably your best choice.

Other good free anti-virus / anti-malware software includes the following. ZoneAlarm incorporates spyware detection and removal software.
Avira AVG ZoneAlarm ClamAV ClamXav

I used to recommend Avast!, but then it was fined US$ 16.5 million for selling users' browsing data.

As for non-free products, I have been very impressed with Kaspersky products. My mobile phone runs Android, and I have been happy with the free entry-level Kaspersky tool for Android. I would use Kaspersky anti-malware software on my other computers if they ran Windows. I use Linux almost exclusively, rendering this moot for myself.

The Kaspersky Rescue Disk lets you safely boot and clean infected Windows systems that can't be cleaned with the normal tools.

Yes, sure, if you're an NSA employee who very inappropriately takes home NSA offensive software, and you install that on your Windows PC, on which you had also installed Kaspersky software, then bad things might happen. But that went wrong in several ways at once that were not Kaspersky's doing. See New York Times and Washington Post stories for details.