I'm not a big advocate for antivirus software, only because it starts fights. But, if I do suggest anything, it would be Kaspersky PURE Total Security. However, this is over $70 I believe. But, if you haven't already gotten your Facebook free subscription to security software: https://www.facebook.com/security/app_360406100715618 then go for it!
In the AV Marketplace at Facebook, they offer Kaspersky PURE Total Security for free among others.
Otherwise, I would recommend Avast! Internet Security as an alternative. There is a $10 off sale going on right now.
Best of luck.
Be sure to keep your product key in a safe location.
Once you do get a security product from the AV Marketplace, you can no longer get a free subscription to anything else. Those are the rules they have in place.