You can recover your PC from Petya.Petya appears on startup like "chkdsk".Before of "chkdsk" screen, you must do that steps:
Step 1: Make a Windows boot disk (USB, DVD etc.),
Step 2: Open your PC via boot disk.(USB,DVD etc.)
Step 3: Format your PC, but don't erase disk. Windows will backup your files in Windows.old folder.
Step 4:Enjoy it!