generally they don't do much either way.
just get Microsoft security essentials and maybe the free version of malwarebytes and spybot. other than that, just make sure you don't go to shady websites and download stuff from them. try going through this: http/www.tomsguide.com/us/antivirus-software-pay-or-free,news-18570.html
tl;dr: For the rest of us, many of the free packages are sufficient....the mere fact that you are diligent enough to download and install a free antivirus application means that you're also probably more careful than most people online and may not need the added protection... You aren't likely to open links in strange emails or fall for false ads on malicious websites. In other words, you're not in much danger to begin with.