The information in this FAQ are based on this article: http://www.malekal.com/2012/01/10/virus-gendarmerie-activite-illicite-demelee/ The ransomwares propagate through malicious advertising (Malvertising) streaming/downloads websites etc..
These ads lead to malicious WEB exploits which aim to automatically infect the visitors.
PCs with outdated software (Adobe Reader/Flash, Java) are more vulnerable to this type of infection.
Hence the importance of keeping your software up to date.
The "Hadopi" ransomware exists in two variants Urausy and Reventon.
The Urausy variant - normally safe mode does not work.
This variant is characterized by the presence of the handcuff image
This variant normally prevents booting in safe mode.
The Reveton variant - handcuffs image is replaced with a WebCam image.
This variant doesn't block access to safe mode.
Safe Mode (Reveton variant)
Restart the computer in Safe mode with network support.