Keep in mind that the newest viruses tend to spoof the originating address. So it LOOKS like you sent something, when in fact you didn't send a thing.

Also, (though I forget which one it is, I think it's Mydoom/Novarg), there's a virus currently out that acts like it's an undeliverable message error, with an attachment.

Most likely, you don't have the virus, if your antivirus software is up to date. (Don't rely on it running in the background though, do a full scan to be sure)