Norton AV has a bootable USB stick for system scanning.
Also you can not trust a USB stick/portable app antivirus to scan a running system because an active rootkit will just hide all of its activities from the AV, and the AV product will never find anything!
You have to remember that the virus people always have a jump on the AV people. Most of the good virus writers, scan their viruses with a dozen or more AV products to make sure they don't detect them.
In this 0-day period there is no protection against them, or de-infections tools. Even when a virus is detected it may damage the files in a non-recoverable way. Only files altered in a known formulaic method are fixable.
You are better off with a firewall to monitor and detect traffic to unusual locations. Even then virus authors are encrypting the viruses, and changing the key so that they don't match the pre-programmed signatures that most AV depend upon.