I use a linux-based distro called "Ubuntu". No viruses as long as you only install from approved repositories. So, I have no need for anti-virus. If you're bored you can try it out. I've been using Ubuntu for a little more than three years now.
Kaspersky is not free, but is in the top 5 worldwide anti-virus. What I do is buy the edition from the last year around July because is a lot cheaper, and the pack offers me 3 license for 3 differents PCs. The price of the last year edition in around 20$.
Provided you are not an idiot with computers and the internet, Microsoft security essentials is all you will ever need. It's one of the best virus defenders ever made, and its very light on resources at the same time.
Wow. I did a full system scan with MS Essentials the other day, after 9 hours of scanning I said eff-it and closed it out. The progress bar had stayed at about 80% done for 3 hours straight, but the "Scanning: file name" was still constantly changing so I guess I had a lot of stuff in that particular area. Maybe I just had a bunch of stuff that needed deleting or something and that is why it took so long but good god that was a ridiculous amount of time.