AVG Free Edition
http://free.grisoft.com/ww.download-...s-free-edition
Why? Because it's light (doesn't stall your OS while it updates) and it updates very frequently, a couple times a day sometimes.
It configures itself with the Windows OS and shields email, etc...
Because I've never gotten a virus, while many people I know who don't use it have.
I don't know why I would/should buy the paid version, but that might be because their marketing dept. is weak... but that's no reason not to use the soft lol.