worzel
If your system is really clean (how up-to-date is your virus scanner?) then these messages are actually coming from somebody who happens to have your address in their addressbook, which has been siezed by the virus and used to forge the sender address in outgoing copies of itself.
It may be possible to work out who this is by reading the mail headers (Outlook doesn't display these by default.) Email me if you need further help with this.