E-mail virus Power Eraser cant fix 219 Spam Mailer Failure Notice

I run Vista. Have Yahoo e-mail.  Four times I have a large number of MAILER FAILURE NOTICEs in my spam box.  I have run complete system scans each time after emptying my spam box.  Have had low threat cookies which were deleted.  I have run Power Eraser 3 times.  The first 2 said low threat problem which were fixed. The last time 219 notices were in the spam box, I ran power eraser and it said a bad registery and it was fixed.  Still I keep getting these mass messages not all the same quantities. My contacts said the e-mails from me have no message, just a link to click which I tell them to delete " DO NOT CLICK LINK".  I am not sure how I got infected, I am cautious to not click unknow links or open unknown e-mails.  There was one e-mail from a contact that was hard to delete and it was odd, it didn't have a message so I tried to send it to spam and them delete it from there.  That person said they had been hacked twice before with a take-over virus, but it was fixed.  Don't know if that is how it got into my system or not.  Anyway have run all the Norton 360 programs, they all seem to work, but still this keeps happening.  I have deleted all my contact list, but it still seems to happen.

ANY IDEAS OF WHAT TO DO??  I JUST WANT A SAFE SYSTEM AND NOT TO INFECT ANYONE ELSE.

   Rosana