Computer Question

So, I recently helped my sister pick out a new iMac. She'd spent the better part of the last decade using Windows machines, but she wanted something that would not take up a lot of space on her desk, and would be easy to use for Web, email, basic word processing, and transferring photos from the digital camera. So I felt pretty comfortable luring her back to the fruity side of the computing divide. (I'm always a little nervous when I persuade people to make the switch - as much as I love the Mac, I know it's not right for everyone.)

Anyway, things seem to be going swimmingly, but she sent me an email about virus protection software today. And I realized that I used to run virus protection software (Virex) on my Mac, but they stopped making it, and I've completely neglected to replace it with anything else on my new computer. So, I know there are a few other Mac users out there on my friends list - do you run virus protection software? If so, what kind?