Viruses (worms) such as this one are usually not sent from the computer of the person whose email address appears as the sender. Here is a quote from the link that Tiara provided.

"Upon activation — usually when a recipient clicks on an e-mail attachment — the rogue program searches though address books and sends itself to e-mail addresses it finds. It chooses one as the sender, so recipients may believe the message comes from someone known."

A couple of years ago, another worm was making the rounds (Klez). People on TER Boston were accusing each other of sending the virus. Wherever it's coming from, you can assume that the person doesn't know that their computer is infected. For instance, you could get this virus in an email from me. However, the email probably came from another person's computer who had both of our email addresses in their address book.

I've attached a link to Norton Antivirus' (Symantec) page explain this virus and the removal procedure.