THIS. The whole "oh some malware got on his USB stick and it autoran on your computer" is sooooo unlikely, that is not how most malware is spread whatsoever, and most people have USB autorun off, I think it's off by default in most modern Windows installs.
Wrong, malware is spread on USB sticks and very often the autorun is turned on on default, this is one of the ways Stuxnet, Flame, Conficker and others spread onto so many computers.