So I have been watching this while Petya / NotPetya Ransomware / Ransomworm story since yesterday morning and trying to get solid information on it, which has not been easy. It now seems that if you did the Microsoft Security Bulletin
MS17-010 - Critical patch from back in March you should be immunized. We did report yesterday that the Petya/NotPetya did not have a built-in killswitch, but one enterprising security researcher from Boston has found a fairly simple way to usurp the file encrypting madness.