Be wary of any pop up that you do not expect especially if your currently opening web sites or reading emails. Be especially suspicious of those that do not carry the name of your anti-virus software and suggest you install their protection. On the other hand there are a small number of free for home use anti-virus packages which are perfectly legitimate. For many years I have had AVG installed on my Mothers computer. There are a few drawbacks to AVG. The first is that it will error on the side thinking some programs are viruses when they are not: often this applies to copes of install files which I tend to keep on a separate drive for re-installation if required. The second is that it can be quite difficult to find the new free version when they upgrade their product, leading you down the paid path to their paid version. The third is repeated notifications: these can give the non IT literate the impression that it is essential to upgrade to the paid for product.
In this respect AVast is a much more honest product. They are both good. The only reason AVG is mentioned first is that it's been on Mum's computer for longer.
Some Anti-virus packages are far larger than they need be. They can seriously affect the performance of your computer without providing any more protection. Both AVast and AVG are reasonably light weight, do not significantly affect performance (except when actually doing a daily scan) and are as good or better at protecting your computer than some paid for products.
If you are an 02 broadband user you will get free McAfee licences, these can be for both home and office use.