A free anti-virus program will be as good as a paid-for one, simply because the free ones are actually parts of paid-for suites of software anyway. (i.e. the company gives away the core anti-virus component, in the hope that some people will then go on to buy the 'bells and whistles' version).
Reading over a decade of posts on here suggests to me that Avast is currently the favourite freebie among ABers (and it's what I use myself):
https://www.avast.com/en-gb/index
If you top it up with a decent anti-malware program, such as AdwCleaner, (and remember to run that program every so often, perhaps once per fortnight) you'll have all the cover you need:
https://www.malwarebytes.com/adwcleaner/
(AdwCleaner is 'out of the same stable' as the better-known Malwarebytes Anti-Malware, and therefore just as good, but doesn't get you involved in a free trial of their paid-for product, which slows some computers down).