Similiar issue for me. Was hit by PCeU Ransomware today, which was undetected by Avast and locked up my PC trying to force me to pay 100 GBP to unlock.
Fortunately I was able to remove it by booting up in safe mode and manually removing the malware trojan following instructions gained from the internet which I accessed via another PC.
Extremely dissappointed Avast failed to pick up on this, particularly since research has found this malicious application has been around since February 2012, a good 10 months.