Explaining Online Virus Safety to Parents?

DrRobin asks: "For the past couple of years, some of the techie parents in my town (Needham, MA) have been giving talks for parents of local elementary schools about online kid/family safety. I know there are a lot of weighty issues here in general but I want to ask if people here have any specific thoughts about the sub-topic I am supposed to cover: viruses (and worms, trojans, etc.). The parents are pretty sharp in general but not necessarily clued in to the net world. I have been on the net since '83 but my principal expertise is the little bitty wetware viruses that make you ill, rather than the software kind that infect computers. Any pointers people think would be helpful to parents would be appreciated."