ClamAV and ClamWin (frontend for windows) are FOSS software, however do note that Clam does not offer real-time protection. For Linux it does offer On-Access scanning, this additionally needs to be setup. I am unaware if the current Windows version(s) has changed this.
Personally antivirus is more of a scam these days. If your using Linux, Android, MacOS or iOS your fine. Maybe you can benefit a little bit with one on Windows.
Just have good security practice. Do not go to sketchy sites, do not download sketchy files and especially do not install and run sketchy things. And do not weaken the built in security of your system like disabling the firewall for example.
In the 1990s, 2000s and early 2010s there was a case for antivirus but computers have come a long way since then.
Besides anti virus programs are mostly closed source and have full access to your device. Allowing them to scan your files and many of these products probabily work with three letter intellegence agencies all across the world to collect data from consumers of these anti virus products.
There has been some rumors that anti virus companies have created malware and released it to the world to drive up sales and demand for their products.
There are exploits that can be installed without a mistake made on the users part, the user can make a mistake, and almost every user downloads and open files regularly.
Windows is less secure than the other options, but the other options are not impenetrable. The biggest botnets are made of Linux IoT devices, and nobody opened the wrong email on they're thermostat...
What a virus scanner will do is check your filesystem and possibly program memory for known footprints. A tool like this can save you from becoming a node on a botnet or being crypto locked. More importantly, if you work from home it can save your company from this issue as well!