Your antivirus is a very important piece of software. It monitors your computer (and some also monitor your online activity) to make sure there is no unwanted activity. Unfortunately the bad guys are always trying something new, and the antivirus writers have to try and keep up, so always make sure your antivirus is up to date (most of them update automatically in the background).
There are many antivirus products on the market, so how do you know which ones are any good? The best thing to do is look for reviews online. This helps weed out the dodgy ones that pay the search companies to come top of your results, because these products are usually vastly inferior and often more expensive than the better ones (some of which are free). The bad ones can sometimes even give you false results suggesting you have a serious problem that only their paid-for product can fix!
Feel free to get in touch for advice and information.