Most new computers come with a free trial period for some Anti-virus program. After all, you wouldn't want to be without protection from viruses, spyware, adware, and the like. But the catch is the "trial period" part of the deal. It's not really free, it's just free for a couple of months or so, if that. Then if you want to keep being protected you have to pay so you can get the regular updates and stay as safe as possible. And you get to pay again each year, with most. Well wouldn't it be nice to get free antivirus software for as long as you wanted it? ... You can!
There are a number of Anti-virus programs which are free for home use. If you want to compare the quality of the free programs versus the pay per year programs you'll have to do some googling. What I can tell you from personal experience is that I've been using freeware Anti-virus programs for years on my Windows PCs and can't remember getting a virus yet. Here are several links to Anti-virus programs. Be advised, they will try to entice you to purchase their non-free versions.
NOTE: If you really want to stop worrying about viruses, use Linux instead of Windows. I've been using Linux for over nine years, never installed any Anti-virus software, and never been infected by a virus (or if I have, it never did anything).