.. what's that thing called .. erm .. arghh.. .... o yeah ..
"Being savy about the internet".
Get's you no virus' at-all. (not had one unintentionally for decades)
Installs for other people, you don't need to buy anything. Microsoft's Security Essentials is fine, and one of my historical free-bees would be FreeAVG.
.. but if you're not savy - your computer doesn't stand a chance, it's a matter of time.
Don't install any antivirus on my gaming computer and nothing on the macs.