I have been using Avast, like many others, very successfully for years. Be warned that, like all suppliers of free software, they have a paid version to promote which may or may not be better than the free offering. You do however have to be careful when on their website to click on the correct option to get the freebie. Naturally they tend not to make it too obvious as to which the correct button is!
My wife's new computer had three months free Norton which she immediately opted for. Two and a half month's later it was screaming like mad to be renewed, fortunately the appropriate Norton removal tool silenced it and Avast was then installed to take over. I havn't dared to do a registry scan on her computer yet to see how many "Norton" entries remain.
P.P.