You should know the basics of computer security before using one, and by basics I mean BASICS. You shouldn't be opening exes from someone offering you a free game, you shouldn't be clicking links in your e-mail from people you don't know, and you should be scanning anything that appears dodgy. What people don't seem to realise is, that they put all their important information on their PC, then completely ignore that they have to take care of it, as if the computer is a magical invention that no-one would want to break into. I think part of the problem is people saying "we should make PCs accessible, so they don't need to worry about security", because they don't, and when a new security issue appears, they aren't paranoid about it, and scanning it, they're just going "oh well, my scanner didn't pick it up I guess it's safe to open".
Whenever you use something that has got personal items stored in it, ie a safe, your car or your house, you are automatically aware of security, it's common sense. Has anyone ever told you to lock your car and wind up the windows since you were ten? But you do it anyway right? Why? Because you don't want someone coming across to your car, opening it up and nicking the stereo. Having a moderately secure computer is pretty easy, but taking away the learning of that concept makes people more vulnerable in the long run (eg when a new virus comes out and their scanner doesn't pick it up).