You're opening a can of worms here, friend.
There's lots of free ones and you're about to see a lot of "this is the best", and "no, you're wrong, this is the best!" So, let me preface my opinion by saying that there's many out there, and most are good. But, only use one single antivirus. More does not equal better protection. Multiple AV's will usually conflict with each other, cause problems, and make each other miss stuff.Now...
I personally prefer Microsoft Security Essentials. Lots of people don't recommend it anymore because "it failed antivirus certifications" (recent news), but I still stand by it. It's fast, free, and good. The main downfall is that it's better at retroactive removal than proactive protection.
http://ninite.com/essentials/
Other good free ones: Avira, Panda Cloud, or Comodo. Avast is great too, but it's gotten naggy and bloated for my taste.
All that said, the best protection is to not rely on an AV to be your "end-all-be-all" protection. Think of it more as an extra precaution, and your main defense against malware should be good online practices. Don't go to any shady websites looking for warez/porn/whatever the lingo is these days. Keep exploitable apps and runtimes up-to-date, like Java, Adobe Reader, Flash, etc. Install Windows Updates. Use alternate browsers like Firefox/Chrome/Opera, and be sure they update themselves. Enable "click-to-run" for things like Java and Flash, so that drive-by infections are less likely. Install and run an on-demand anti-malware tool like Malwarebytes, and run weekly or monthly scans to keep tabs on things that may have slipped through undetected.
http://ninite.com/malwarebytes/