For free, bitdefender free. Bitdefender is on par with kapersky and the free version uses the same software as the paid just less customization, so you get the same protection.
For free: Avast is very competent. For Paid: Kaspersky and Nod32 are both good paid options.
The difference between free and paid usually isn't protection levels but other features such as sandboxes and other tools on top of the usual shields and scanners.
Try Gdata Internet Security, it's the best AV out there in my opinion. Pair it with Malwarebytes Antimalware, also try adding Adguard and WOT to your browser. Do this and your system should be secure from almost every virus out there.
Panda's free AV is really good as well. I switched from Avast Free to Panda after the most recent versions because Avast isnt as good this year as it was 2 years ago. Paid - Bitdefender.
Generally speaking I dont run local AV in Windows, but I will run a firewall. Instead I just scan my system every so often with ComboFix and JRT from http://www.bleepingcomputer.com.
I practice common sense on any platform. I like to have a solid AV on my PC as its more likely to fall pray to something dodgy at anytime regardless of best practice.
I don't feel the need for it on the Mac. I only mentioned the Mac to see if any Apple haters crawled out of the wood work. For a laugh!