Yeah, it could've come from anywhere. Those viruses are very common. They infect your computer, and you never know you have it; then your own computer randomly sends the virus to everyone you know, without you knowing about that either. And it forges the source address to be from someone in your address book, so that people are more likely to open it. That's how it spreads quickly.

If you avoid using Outlook / Outlook Express, you'll be much less likely to get those kinds of viruses. Also, never ever open an email attachment, even if you know who it's from (or who it LOOKS like it's from), unless you specifically know that someone is sending you something, or if the email is personalized to the point where there's no doubt who it's from. Don't even view it in a Preview pane. Disabling the Preview pane entirely is a good idea to be honest.