It would have come from an email attachment that you opened/clicked on, most likely.
Every email leaves a secret "fingerprint" if you look at the headers. It'll tell you which mail server processed it, the name of the PC it came from, and the IP address of where it originated. This info may or may not be useful.
Most likely it was some poor soul in the same situation as you, who clicked on it unknowingly (without a virus scanner). These things often infect your email program, scan the CONTACT LIST, and send the virus back out immediately to everyone in the list. (disguised as a friendly message from YOU; someone they know).