Innocent users told: "Pay up or we tell the police about your child porn"

While criminal gangs are more widely associated with threatening denial of service attacks unless they get a kickback of thousands of pounds, it seems some are taking a more small-scale approach to extortion: now average PC users are being targeted.

Instead of £50,000, the criminals are making demands by email of a mere £50. Unless they're paid off, they threaten to tell the police about the child pornography they've installed on your machine.

There's no pornography there, obviously, but the threat and the (relatively) small amount of money involved will no doubt get a few people who haven't been exposed to the usual internet scams to reach for their wallets.

The 'child pornography' threat email follows the pattern of a scam of a few years ago where the criminals would threaten to take over a network or install a virus on it unless they were given $20 or $30.

Richard Starnes, president of the security professionals association ISSA UK and director of incident response at Cable & Wireless, said the emails looked to be originating from Russia and Eastern Europe, and bore the hallmarks of organised criminal gangs.

Criminals going after individuals rather than companies isn't exactly new - phishing has now become commonplace in the internet crime lexicon - but the emails mark a move away from trying to steal money with deception towards outright extortion.

"They've milked the gaming industry a bit too much," Starnes said. "[They are] expecting it and they're watching out for it now." Some home users, however, might not be quite so internet crime-savvy and could provide a rich seam of potential victims - and profit - for the criminals. "Organised crime is a business - they've had an idea of how to change their market."

For users finding similar attempts to scam them landing in their inbox, the advice is to step up security and treat the email like any other attempted crime.

A spokeswoman for the National Hi-Tech Crime Unit (NHTCU) said in the first instance, anyone getting one of these scams should report it to their ISP and block the email address. Reporting it to local police is also a good move, according to the NHTCU.

As in the recent case of the Russian extortionists arrested this week, some people will pay up rather than risk the wrath of the gangsters. The NHTCU spokeswoman said people should talk to the police instead of keeping silent. "It's blackmail, at the end of the day. We need to know what's happening so we can investigate. If people don't report it and it continues, that doesn't help to resolve the problem," she said.