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updated 4/12/2011 9:49:17 AM ET 2011-04-12T13:49:17

Online scammers are calling on a tried-and-true trick to put cash in their pockets and leave victims helplessly stuck on a long-distance call to Africa.

Researchers at the security firm F-Secure have come across a Trojan that automatically locks Windows users’ screens, taking them over with a message that reads, “You may be a victim of a fraud or there may be an internal system error.”

To unlock the screen and escape control of the hostile malware takeover, users are forced to “complete activation” of Windows, a process that involves dialing one of six different long-distance phone numbers.

The message on the now-frozen screen claims the call will be free of charge, but it’s not, and victims end up on hold for several minutes. Meanwhile the creative criminals – who are routing all calls to call centers they’ve set up in Sao Tome and Principe, Denmark and Madagascar -- collect the long distance charges victims rack up.

Making matters worse, each call is billed at a higher rate than a call to that location would ordinarily cost, a criminal ploy called “short stopping,” according to F-Secure chief research officer Mikko Hypponen.

After a period of time, victims are given a six-digit code which unlocks their screens. (In case you come across this scam, the unlock code is 1351236).

Unfortunately, these types of ransomware scams – in which scammers demand money to fix a computer problem that doesn’t exist – are not uncommon.

Last November, a dangerous variant of the infamous GpCode ransomware made its way around the Web, in which the program not only demanded payment, but actually hijacked computers’ master boot record, making it impossible for victims to use data-recovery software once they regained control of their systems.

Trojans can infect your computer in a number of ways. Infected email attachments, bogus security or video software and infected Web pages are the most common methods. The best defense against Trojans is to have antivirus software and keep it constantly updated.

© 2012 SecurityNewsDaily. All rights reserved

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