Petya, a new form of ransomware, locks victims out of using their data much like other forms of ransomware. The difference is that Petya uses dated programming techniques to do so. Rather than scrambling data files, Petya scrambles the index that your computer uses to define and make sense of the data. Even more disturbing, Petya normally comes in a harmless looking Windows Program file that users unknowingly end up running on their machines. Read more about this ransomware here, at Naked Security by Sophos.