It's using your email address as a 'return path'.

Call your Internet Service Provider and ask them what to do. This happened to a friend of mine, and her computer had a virus. In her case, she went to too many sites and downloaded cute greeting cards and all - and that's how the spammers and viruses came in.

You don't do that, I know....just that it can happen to anybody.

Can you display Full Headers on the returned emails? That might give a clue.