A new strain of "Ransomware" that attempts to coerce victims into paying $35 to unlock their Windows PC, is doing the rounds.

The scam uses a variety of premium rate numbers in different countries, and UK regulator PhonePayPlus is investigating the suspected misuse of a type of premium rate line normally used for sex lines in the UK.

The Delf-CTK Trojan poses as a "Browser Security and Anti-adware" security application whose license has expired. Windows machines infected by the malware are confronted by a full-screen message that poses as a Windows error. Ironically, but unsurprisingly, the malware typically uses Windows exploits to infect vulnerable machines.

Prospective marks are invited to call a country-specific premium rate number and enter a PIN to obtain a license code. The US premium rate number belongs to "passwordtwoenter.com", a payment processing firm used by hardcore porn sites, according to anti-spyware firm Sunbelt Software, which was the first to warn of the ruse. Passwordtwoenter.com is registered to Global Voice SA, a firm based in the Indian Ocean island state of the Seychelles.

If the US number doesn't work, prospective marks are invited to call alternate numbers including a satellite telephone number and another in the West African nation of Cameroon, Computerworld adds. UK and French premium numbers also feature in the scam.

The 0909 number British marks are invited to call is reserved for adult premium rate lines, premium rate regulator PhonePayPlus told El Reg. PhonePayPlus agreed to investigate the issue, after we told them about the scam. A spokesman added that he wasn't aware of previous UK cases where malware has been linked to attempts to prompt users into phoning premium rate lines.

Ransomware packages (which began appearing early in 2006) typically use malicious code to gain control of user files, encrypt them, and threaten users that they won't see these files again unless they hand over a cash "ransom" to hackers.

The Delf-ctk Trojan is more subtle than this, the demands are less transparently hostile, and a different (more advanced) payment method is used. Users infected by the malware are locked out of their whole system by malware that takes over their desktop - not just preventing them from opening particular files - so in some ways the Delf-CTK Trojan is nastier than earlier ransomware strains such as Gpcode. ®