First, those email are most likely not from your computer.
It can be from anyone, anywhere.. also may be send by/from your friends/families computer that have a virus.
It is annoying, but that how spam/junk mail works.
You can not do anything...
My yahoo account has been sending random emails to people I haven't contacted in over a year - they are not even in my address book. I'm trying to see how this could be happening from my computer, but it doesn't make sense, and I've run lots of anti-virus software and other tools (reply emails I have had from annoyed recepients say the name of the virus - it's the W32-NyxemD). Is this a problem with yahoo, or is it indeed my computer? What can I do to stop it? It's quite annoying and embarassing but hasn't caused too much damage - yet. I use Firefox and Windows XP.