While breaking into your email account at your ISP isn't out of the question, far more often than not is that your email address is being spoofed by a spammer, or someone you know running Windows has been hit by one of many email viruses that automatically send out emails from their computer to everyone in their list and randomly choose of of the addresses in their address book as the "sender".
With spoofing, spammers pick any ol' known good address from a list of thousands, or millions at their disposal, and then send out tons of various pieces of spam with that address as the sender. They only do it for a few days or so to prevent automated systems from blacklisting that address, then some other victim's address becomes their next "sender". The return mails are from the server's they send the spam to that are dead accounts. The non deliverable message comes back to you rather than the spammer because your name is in the sender field. There's nothing you can do about this. The spammers will move on to another address and the flood of returned messages you never sent will stop.
For email virus issues, any persons you know running Windows who would also have your email address in their contact list would need to check for and remove any viruses that may be on their system.
