Trojan Horse?

Every time I run my Norton this "trojan horse" (perhaps it is something else, I am not that knowledgeable) sends fake e-mails to everyone in my address book.  I run both Norton and one other program every other day and I keep getting messages saying everything has been found.  Even ran Norton Eraser which said it found nothing...guess what?  I just had 25 e-mails returned to me - everybody has removed me from their list.  Can anyone tell me what to do about this? And what it is?

 

Thanks.