Computer viruses or malware can be defined as a piece of code which is capable of copying itself and typically has a detrimental effect, such as corrupting the system or destroying data on the system. Viruses can compromise your personal data (including financial data), take over portions of your computers processing power to run unseen applications in the background, encrypt all of your data and literally hold that data hostage unless you pay a fee to unlock it.
If you use the internet for anything (e-mail, simple web browsing, zoom calls) your computer should have anti-virus, or network security software installed on it. However there are many free, good, solutions available that are perfectly sufficient for casual, low risk, internet usage.
Avast One Essential - Top rated by PC Magazine Avast is the only free software on this list that also offers a software firewall as part of it features (A Software Firewall is a piece of software that is installed on your computer in order to protect it from unauthorized access)
AVG Antivirus Free - AVG Antivirus has a large fan base built up over the 30 years it has been in existence. Recently AVG was actually purchased by Avast, and they each use the same antivirus "engine" to provide security.
Bitdefender Antivirus Free Edition - Bitdefender excels at spotting fraudulent web sites that try to trick you into giving up your personal information (known as phishing).
Sophos Home Free - Sophos is a big name in business and enterprise antivirus, endpoint protection, and security suite products, but you can get much of the same enterprise-level protection for your own devices, in the form of Sophos Home Free.
Windows 10 has built in Antivirus - sort of
Microsoft Defender Antivirus scored 3.5 out of 5 stars in a PC Magazine survey. As a part of Windows it is technically "free" if you are already running windows. Any of the antivirus utilities listed above offer more layers of protection (although Microsoft Defender does offer a built in firewall). But Defender protects everyone who can't be bothered to install a third-party antivirus tool (as long as they turn it on and keep it updated).
Just do something!
The list above is far from exhaustive and there are several other good free choices available (Kaspersky, Avira, Adaware, ZoneAlarm, Malwarebytes). The difference in performance between any of these free solutions will only vary by 2 or 3% in speed an effectiveness. Choosing one simply because you like the way the interface is laid out better than the others is just fine if it makes you more likely to use it and keep it up to date.
If you have a favorite piece of Antivirus software I did not mention, please feel free to tell us about it the comments below.