Not every windows user is as saavy as you. Nor are Mac users. My point though is that as a casual computer home user, you WILL be more likely to have issues with viruses if you use Windows machines. I know 10 people right now whose machines are running slow because of the numerous viruses they have unknowingly picked up. Not that Macs don't have issues, just not due to viruses.
Anyone who "unknowingly" picks up a virus these days is fiddling with the default settings.