NEW RANSOMWARE
Today a new ransomware appeared on my computer that said in Hungarian "A számítógépe a rendőrség által blokkolva lett", in English "Your computer has been blocked by the police". I couldn't close it or anything, they wanted me to pay about a hundred bucks. Do not pay!
My Avast actually tried to block this virus, also gave the warning sounds but failed. I ran it in Safe Mode but it didn't find any threats. If there's anyone reading this from Avast Support, maybe you should consider including this virus definition in the next update.
SOLUTION
The programs on youtube removal manuals are useless. Run Windows in Safe Mode (restart computer, press F8 before Windows loads, and select Safe Mode with Prompt) and restore your system, that solves it. Since Avast and apparently any other anti virus is not able to deal with this, do not visit the website where you got the virus hopefully only till the next update.
Cheers