There are two potential issues, you may have a virus on your machine or your e-mail account password ***** have been compromised.
Can you advise what anti virus you have installed on the PC you use to send e-mails from, and who your e-mail provider is?
I suggest you change your password ***** a matter of priority, and use something completly different eg if your password ***** Welcome1 dont use Welcome2.
Ok, can you confirm your Norton anti virus is up to date, and how you send e-mails (is it via the web browser or via a email program such as outlook)?
Ok so you sending e-mails via webpage, eg by going to yahoo.com logging in.
This would most likely rule out a virus on your PC, a virus on your PC would most likely only affect you if e-mails are being sent from an application on the PC such as Outlook.
When you go to Yahoo, in the 'Sent' folder, do you see the e-mails which are being sent from your account without your knowledge, or onyl the ones you compose?
Ok this would suggest someone has managed to obtain your e-mail password ***** send e-mails from a third party product.
By changing your password ***** should stop any further e-mails being sent from your e-mail without your knowledge.