AVG
http://free.avg.com/us-en/homepage
Ive been using it for years and never had a virus. Except one time and that was my own fault, I took a gamble and I lost.
1) It has frequent virus definition and program updates. Every 9 or 10 months they come out with a new version of it so your not just using a free version from 5 years ago.
2) Has a game mode if you do pc games.
3) The program has a very small footprint so it eats up barely any system resources.
4) It doesnt have shit popping up all the time that get in the way. It only alerts you when you have a issue or there is a potential issue. It doesnt even popup asking you to buy the full version or donate any money ever.
5) It reads everything coming into your pc silently. So if you go to log into a website that has a virus in it avg will stop the page loading before it saves to cache and warns you giving you a option. This is again only for real threats, it wont pop up everytime you log into like ebay because a java script for the websites menu bar starts loading.
6) For being free it has a comprohensive list of options in it that allow you to chose a lot things.
7) Unlike some free av's it has a email scanner in it and it has nice option sets to it.
8) It just works really well. I run it on both of my pc's, the pc's that I built for friends and no one has had a issue. System scanner on my Id guess about 500gb of data and the options set to scan every possible file it only takes about 40 minutes to scan it all, the first scan takes longer since it optimizes itself to scan faster next time you use it.