Windows XP - Blaster Worm in 2003 (http://en.wikipedia....(computer_worm))
Back in those days, a fresh install of Windows XP would get the virus within about 12 minutes of being connected to the Internet (just sitting there). That's what forced Microsoft to put out Windows XP SP2 with an extra feature of a firewall. Traditionally before then (and after that too), Service Packs did not contain new features. They were only bug fixes. The only other "service pack" that has new features was Windows 8.1.
Oh how I remember. I always did a clean install of my OS before going back to College and after XP finished, I kept getting the forced shutdown. I did a second re-install and the same thing happened. So I went online on a second computer and read about it.
The best part was, the next week at College, when we hooked our machines to the network, we got to a landing page that said they had to update antivirus and install patches. Obviously no one did it and our network at school was shutdown for almost a week. Good memories.