No anti-virus product (AVP) can ever be 100% effective. New viruses are being created every day and the people that make them obviously make sure they evade detection especially by the most popular AVPs.
Norton & McAfee are the most popular and have a very large share of the market. These products are sold by giving them to computer retailers and other companies (e.g. PC world) with a payment made when a customer renews the (free) initial license. It is obviously in the interests of these companies to persuade you to accept the AVP they are providing on the new PC you buy.
In fact, Norton and McAfee tend to be at the bottom of the effectiveness league for AVPs primarily because they are targets for the people that make the viruses.
There is no reason to suppose that paying for an AVP will mean that the protection is better. Indeed, often the additional software added in a paid version adds no benefit but affects your PC performance adversely so that the virus may not be dealt with as quickly!
The plain fact is that you are far more likely to prevent a virus by being careful about what you open, download or click on in a web page. Don’t even assume that because you know what a product is that you are safe! A common practice is to copy some free software, bundle it with a virus and make it available for download on a site that looks exactly like to real site. Always check where you are downloading software from as well as what you are downloading. In particular, avoid registry cleaners, driver updaters and any other software that claims to speed up your PC. It will not do this (if anything these decrease speed), and these products are very commonly used to convey viruses.
But there are very many viruses out there so you need to have an AVP. You need to make sure it is running by checking the history logs regularly and you need to make sure it does scan your PC automatically or do it yourself.
So, which to choose? The effectiveness of AVPs varies from month to month so it is very hard to say which one is the best. Microsoft security essentials (now renamed windows defender which must not be confused with what used to be defender in windows XP) is the current choice as this does not bother you with constant adverts. This product has been included in Windows since windows 8 so all you have to do is remove whichever AVP is active and it will popup automatically for you (if you are a sceptic you might wonder if there was a ‘backhander’ given to ensure your PC came with another one loaded). I used to prefer AVG but lately they have caused quite a few problems. The argument to avoid replacing Windows Firewall is even stronger. Microsoft require registration of software to allow it through this firewall (included in Windows since XP). Any software blocked is allowed by you at your peril. Other vendors have to rely on their experience and ask you the user if the software is unknown. Which do you think is best?