THAH, that´s typical
ransomware, it´s a nastier type of virus... I remember a similar case where I suddenly had a similar window from the austrian police and some stuff I should pay to them to unlock my pc. Best part: they added some company logos to make it look more real but I knew it was fake because our police isn´t sponsored by our SPAR markets...
Just google for a solution (or use Nash´s solution), ransomware is annoying stuff but there´s almost always a working solution on the web for it, easy to find