The malware causing the damage is called Reveton, a drive-by attack incorporating the Citadel bank Trojan platform that has caused a since the beginning of 2012. It weaves its spell by threatening victims with prosecution by the US Department of Justice (DOJ) after - it claims - detecting child porn or other illegal activity.
Victims can avoid trouble and regain control of the infected PC by paying a bogus 'fine', an option a growing number of people appear to be choosing.
"While browsing the Internet a window popped up with no way to close it. The window was labeled FBI and said I was in violation of one of the following: illegal use of downloaded media, under-age porn viewing, or computer-use negligence," one victim told the FBI.
"It listed fines and penalties for each and directed me to pay $200 [£128] via a MoneyPak order. The page said if the demands were not met, criminal charges would be filed and my computer would remain locked on that screen."
The FBI's advice is not to pay out under any circumstances and to turn to a security programme or computer support shop to remove the malware.
Unfortunately, for every complaint there have probably been several victims who have paid up without saying anything. Conceptually, ransomware is a social engineering attack that preys on people's fears of exposure (even if totally innocent) or desire to avoid hassle.
"We are getting dozens of complaints every day. Unlike other viruses Reveton freezes your computer and stops it in its tracks. And the average user will not be able to easily remove the malware," said Donna Gregory of the FBI's IC3 unit. Victims , she said.
Earlier this month the UK's Police Central e-Crime Unit (PCeU) about an unidentified ransomware Trojan that had hit over a thousand PCs. This was almost certainly also the work of Reveton, the notoriety of which has surged in a matter of months across the globe.