Agree completely with Anahata.
Whoever reported it to them should have sent the message to them. Your ISP people can look at the headers to determine if it really DID come from you. They should have done so before they bothered you with it.

If they simply believe what anyone tells them and don't know enough about spoofers and viruses to be bothered checking, they're morons.

It really does sound to me as if the e-mail came from elsewhere. I've gotten bounced messages myself, but my ISP has antivirus on both outgoing and incoming stuff, so they'd know it wasn't from them.