To vaccinate your computer so that you are unable to get infected with the current strain of NotPetya/Petya/Petna, simply create a file called perfc in the C:\Windows folder and make it read only.
First, configure Windows to show file extensions. For those who do not know how to do this, you can use this guide. Just make sure the Folder Options setting for Hide extensions for known file types is unchecked like below.
Once you have enabled the viewing of extensions, which you should always have enabled, open up the C:\Windows folder. Once the folder is open, scroll down till you see the notepad.exe program.
Once you see the notepad.exe program, left-click on it once so it is highlighted. Then press Ctrl+C to copy and then Ctrl+V to paste it. When you paste it, you will receive a prompt asking you to grant permission to copy the file.
Press the Continue button and the file will be created as notepad – Copy.exe. Left click on this file and press the F2 key on your keyboard and now erase the notepad – Copy.exe file name and type perfc as shown below.
Once the filename has been changed to perfc, press Enter on your keyboard. You will now receive a prompt asking if you are sure you wish to rename it.
Click on the Yes button. Windows will once again ask for permission to rename a file in that folder. Click on the Continue button.
Now that the perfc file has been created, we now need to make it read only. To do that, right-click on the file and select Properties as shown below.
The properties menu for this file will now open. At the bottom will be a checkbox labeled Read-only. Put a checkmark in it as shown in the image below.
Now click on the Apply button and then the OK button. The properties Window should now close.
While the C:\windows\perfc file is all you needed to vaccinate your computer, it has also been suggested that you create C:\Windows\perfc.dat and C:\Windows\perfc.dll to be thorough. You can redo these steps for those vaccination files as well.
Your computer should now be vaccinated against the NotPetya/SortaPetya/Petya ransomware.