The reason for the question was for confirmation that it wasn't anything you might be doing so we can get on with trying to find the malware responsible.
Was anything found with AVG anti-spyware ?
It is possible that what is sending these emails is hidden by something else and that is why avast and SAS aren't finding anything. This could be a rootkit and the anti-rootkit tools suggested by Tech may be able to remove that so the spambot can then be found.