A powerful form of ransomware, Cryptolocker, has infected about quarter of a million Windows computers, a new report by security researchers has revealed.
Cryptolocker scrambles users' data and then demands a fee to unencrypt it.
According to the BBC, Dell Secureworks said that the U.S. and UK had been worst affected.
It added that the cyber-criminals responsible were now targeting home Internet users after initially focusing on professionals, the report said.
The first versions of Crytpolocker appear to have been posted to the net on 5 September.
According to the report, early examples were spread via spam emails that asked the user to click on a Zip-archived extension.
Later it was distributed via malware attached to emails claiming there had been a problem clearing a cheque.
Clicking the associated link downloaded a Trojan horse called Gameover Zeus, which in turn installed Cryptolocker onto the victim's PC, the report said.
By mid-December, Dell Secureworks said between 200,000 to 250,000 computers had been infected.
