This threat is a .DLL file that displays the lock screen used by variants of the Ransom:Win32/Reveton family.
variants arrive on your PC with a random file name. They download this .DLL file.
The .DLL file also has a random file name with the extension pad in the folder %APPDATA% or %TEMP%. This means the threat can display the lock screen message with or without Internet access.
The message in the lock screen is tailored to you location in such a way that, for example, if, based on your IP address, you're located in the US, the lock screen appears to be a message from the FBI.
Analysis by Stefan Sellmer
Your PC might be locked with a warning pretending to be from the FBI or your national police force.