Keith,

Do you access your Yahoo mail via a web browser, i.e. true webmail or do you use and email client like Outlook, Outlook Express, etc?

If its purely via a web browser I think you will find that you don't have a virus on your PC but that someone has got hold of your login details for your Yahoo account and are sending the spam emails using your credentials.

This happens a lot with webmail based systems such as Yahoo and Hotmail. It can also manifest itself by sending fake instant messenger messages from your account as well as has affected some of the users on this site.....

Try logging into your Yahoo account and changing your password to something new containing both letters and numbers and check whether Yahoo always use secure login page when entering details. Sometimes Hotmail defaults to no security when logging in which means your passwords go across the web unencrypted!

HTH

Sean