Sounds very much like the Klez/Magistr family of viruses. Your email address is being used as the proxy, which is why you're getting the undeliverable messages.
Someone who has your email in their address book is the infected party. Not much you can do except wait for it to subside as noted above. And there is no way you can find out who has your address (with or without your permission). Using a spoof address for bulletin boards and usenet cuts down your exposure, but it's not foolproof.