Define "common sense".
My dad is almost 70, he only uses his computer for e-mail, freecell, and word processing. He is probably the most intelligent person I know, and has been on the internet since pages could take 5+ minutes to load. He runs AVG, as well as regular checks with Malwarebytes/Spybot/etc.., yet he still manages to get the occasional virus/worm.
I, on the other hand, surf the net recklessly poking my nose into the shadier parts of the information superhighway. I have Avast! running in the background, but have never gotten a warning from it, I think the last virus I got was maybe 2yrs ago.
I think that caution only gets you so far, knowledge can get you a bit further, and a sprinkle of luck can keep you happy.