Chances are it wasn't sent from you, Heb -- Most viruses these days forge who they're sent from. It's impossible to say who it was really sent from without digging into the headers of the message.

If it was sent from you to Vegas, though, there's a good chance the infected machine belongs to someone who's either got both of you in his/her address book, or has sent or received mail to/from the two of you in the past.

In any case, I wouldn't worry about it too much, assuming you have an up-to-date virus scanner installed. There's really nothing you can do to stop these forgeries from happening. The best you can do is keep your own machine patched up and virus-free.