I use Microsoft Security Essentials (free download) on my Win 7 machines and Windows 8 includes Windows Defender. There are other free AV packages out there as well, such as Avast.
I haven't used a paid AV on any of my Windows computers since XP was new. Of course the best defense against getting a virus in the first place, is using a little common sense to know what to click on or not click on and knowing what not to download.
Of the paid AV packages, about the last one I would choose would be Norton.