Cheese - there are a number of viruses that will scan your computer for Email addresses and use them to send out Email using their own SMTP engines - so you would never know if your machine was sending out messages, except that it would be slow (due to the volumes of messages being sent by the virus).
Newer versions of these viruses also alter the source address - so the mail doesn't look like it is coming from you. Someone with your Email address on their machine may have the virus, and the virus is changing the source address so the mails it sends look like they are coming from you (among others).
Finally, some viruses originally arrive in the form of a delivery failure message.