E-mail messages containing viruses/worms

I currently have two web-based e-mail accounts. I do not use outlook or outlook express for either of these accounts, therefore, I have no address book locally stored. I have recently begun getting returned e-mail messages that have been sent from both of my accounts to addresses I do not know. All of these messages contain attachements that look very much like the Sobig worm. I have patched and scanned my computer – no worm found. I became so concerned that I actually formated and reloaded all the computers on my home network, I’m still getting the bounced messages. This is also happening with my daughter’s Yahoo account. Anyone have an idea where this is generating from?