I had a similar problem. Somehow they got hold of my address book in Yahoo mail and a Chinese site sent a mail to everyone offering electrical goods supposedly sent on by me. I think it must have been a Trojan or something as it seemed to disappear when Spybot and Adaware was run. But irritating none the less.
On the other hand, on by product was that I'm back in contact with an old friend whose address I had thought defunct. So silver lining there.