If sality comes down as one of the known variants then Avast will stop it. If the payload file is a new variant, Avast may not detect it but it will pick up on the infector at work, and should block it. But it is beholden upon you to be cautious - if you are offered a USB with good programmes on it, scan it first with both Avast and MBAM, that should stop the great majority of nasty surprises
I have just cleaned two systems from one location with Virut. The one protected by Avast stopped the infection from spreading, all I had to do was locate and remove the trigger, job done. The other system was using Norton and we had to reformat as it was too far gone, ove 400 files corrupted. So draw your own conclusions from that