A new subgenre of malware that ransoms your PC is now stalking the internet for victims, says a security developer. The so-called "ransomware" locks up your PC and demands $35 to return control to you is on the prowl, said Alex Eckelberry, CEO of Sunbelt Software Distribution in Clearwater, Florida.

Victims of the Delf.ctk Trojan horse are told to dial a 900 number that can be traced to a payment processor used by hardcore pornography Web sites, he added.

Users infected with the Trojan horse see a full-screen message posing as an error generated by Windows, according to screenshots posted by Eckelberry on the Sunbelt company blog.

"You're completely locked out of the system" after the Delf.ctk Trojan horse installs and runs, said Eckelberry.

Ransomware, a malware that tries to extort money from users after an infection - usually to return access to suddenly-encrypted files - is rare, but not unknown, PC World reported, adding the last significant outbreak was in July 2007 when another Trojan horse dubbed "GpCode" demanded $300 to unlocked frozen files.