I would recommend that you change the password to your Yahoo account. It sounds like someone has hacked your email password and is sending spam from your account. I would also like to see what ESET finds.
DownloadESET online scanner
Click on START, it should download the virus definitions
When scan completes, click on LIST of found threats
Export the list to desktop, copy the contents of the text file in your reply
You may not get a listing if nothing is found