There's a really decent assortment of free anti-virus software available on the Internet these days. Why are they free? It's a form of advertising, a way to "get in the door". Do something well, get positive reviews for it, then entice people to pay for additional features. Some free anti-virus is more obnoxious than others. Some will give you pop-up reports of the last scan results, tacking on offers to upgrade to a paid version. Others will just pop up offers at random, not in connection to any scanning activities.
OK, so if you're on a budget you've got free anti-virus installed. But if you don't need the added features offered in the paid versions, are you just as well protected for the installed functionality? The effectiveness of these programs relies on the breadth and frequency of comprehensive virus definitions, plus periodic updates to the engines for how they perform their scans. The software must know what to look for. If the free and paid versions get the same subscription updates, then they should be equally effective, right? Or... has anyone learned something to the contrary? If you can afford it, is it really best to buy your anti-virus software?