...Avast is free antivirus so i don't think that it is good. A licensened antivirus is good like bitdefender, eset, mcafee, quick heal, etc... are good.
Free does not mean it is not any good. An offer of free anti-virus or anti-malware software is essentially a marketing technique
...a way of advertising and enticement to get folks to try a product and if they like it, to purchase the full (or Pro) version which typically provides more features. Marketing and promotional strategies are built into the vendor's business model as part of their operating costs. Bottom line...it's all about generating revenue and finding new and creative ways to do so
. As such, users may have to deal with occasional nagging pop-ups or nuisance advertising and prompts to upgrade to the paid version.