Good advice about keeping AV up to date (yes I wound say that)
Although people don't always keep up to date with updates to their computer which plug the holes that things like this (It's called conficker) use.
http://windowsupdate.microsoft.com is your friend!
They release major updates on the second tuesday of every month. Your little shield icon in your task bar should do this automatically.
And if you're a smug Mac user - We have databases full of viruses that infect macs! they just don't make the news so much