Well, back when you were still a child, and I was working as a developer for a software company, we had a major problem because there was active malware that spread on Windows NT and 2000 machines without any user intervention.
It brought down networks world-wide.
It is still around, though almost nobody runs those old versions of Windows anymore.
When Windows XP was first released there were several remote-compromise bugs. They have since been fixed, and you have those updates installed so you're safe. But that doesn't mean you'll stay safe. All it takes is for someone to find a bug that affects XP, Vista, and 7 all together, and bang, a worm will arise and infect thousands of machines, maybe more. Maybe yours.