system
2
As you might see in the virus description on our web site, Sobig-F (and many other “modern” viruses) forges the sender’s email address. It actually finds two addresses on the victim’s computer and uses one as a recipient and the other as a sender (some viruses combine the sender’s from two different addresses so it even does not exist in many cases). Wrongly configured mail scanners (usually on the mail gateways) then send the notification to totally innocent people 
If latest avast! (and cleaner as well) says you are not infected by Sobig-F, then you are not
!
Hope this helps
Pavel