See the last link in my signature for some hints and links. FYI for future reference, the first thing to do when trouble like this surfaces is power down immediately without clicking anything. Then try to initiate System Restore in safe Mode.
FYI Malwarebytes free can be downloaded, updated and run in 'Safe Mode with Networking' which might find somthing extra.
McAfee can also be run in a silent mode by right-clicking the taskbar link and selecting Scan.
There is also a removal guide for Metropolitan Police Malware here: http://www.bleepingcomputer.com/virus-removal/remove-police-central-e-crime-unit -reveton-ransomware
If that doesn't help follow the Hijackthis or DDS routine mentioned near the bottom of that last link.