There are some hints and tools in the last link in my signature below. There is also a removal guide here: http://www.bleepingcomputer.com/virus-removal/remove-police-central-e-crime-unit -reveton-ransomware
You also can try
works for RCEU as well as FBI ransonware. Also try
especially Stinger and Malwarebytes.
Have you tried restoring back to before you got this?
Marked as assumed answered as 2 months old. In answer to the question "why..." no antivirus catches these things because of the way they work - involving user intervention, no matter how trivial, to activate.
If you ever spot one of these don't touch anything. Hard power off and then boot into Safe Mode and try System Restore.