Several years ago, something very similar happened to me and it turned out to be a Trojan (well known name but I can't remember what) It turned out to have been sent to me by a friend and then things were mailed out to everyone on her address book. Little bits of emails, junk etc were being sent out.

I am not sure it works because it has never happened again - but I followed a suggestion back then: it said to add an entry to your own address book called "wormalert" or something with a similar warning. In the place of the actual address, have it list your own inbox. This way, if something is automatically using all the addresses you have listed, you should get this mailed back to you. Again, not sure if it will alert me but I do have *worm-alert!! as the first entry in my address book.

Have you tried checking symantec or a similar virus place for a list of current threats to emails? It will give you files to search for that an email virus might leave. That is how I found out what I had and what to do to rid myself of it. Hope you can figure out what happened in your case