This Microsoft guy suggests that Microsoft Vista is so secure it doesn't need antivirus software.

"I'll give you an example: My son, seven years old, runs Windows Vista, and, honestly, he doesn't have an antivirus system on his machine. His machine is locked down with parental controls, he can't download things unless it's to the places that I've said that he could do, and I'm feeling totally confident about that," he added. "That is quite a statement. I couldn't say that in Windows XP SP2."