The scam always starts the same way: the phone rings at someone's home, and the caller - usually with an Indian accent - asks for the householder, quoting their name and address before saying "I'm calling for Microsoft. We've had a report from your internet service provider of serious virus problems from your computer."
Dire forecasts are made that if the problem is not solved, the computer will become unusable.
The puzzled owner is then directed to his computer, and asked to open a program called "Windows Event Viewer". Its contents are, to the average user, worrying: they look like a long list of errors, some labelled "critical". "Yes, that's it," says the caller. "Now let me guide you through the steps to fixing it."
The computer owner is directed to a website and told to download a program that hands over remote control of the computer, and the caller "installs" various "fixes" for the problem. And then it's time to pay a fee: £185 for a "subscription" to the "preventative service".
The only catch: there was never anything wrong with the computer, the caller is not working for Microsoft or the internet service provider, and the owner has given a complete stranger access to every piece of data on his machine.
An investigation by the Guardian has established that this scam, which has been going on quietly since 2008 but has abruptly grown in scale this year, is being run from call centres based in Kolkata, by teams believed to have access to sales databases from computer and software companies.
Often, the victims are inexperienced or elderly, convinced by the apparent authority of the callers and the worrying contents of the Event Viewer. In fact, such "errors" are not indicative of any problems.
Investigators who have spoken to the Guardian say that one man, based in the city of Kota in Rajasthan, is behind the centres running the scams.
He has provided fake documentation to a number of payment companies including PayPal and Alertpay, a Montreal-based online payment company, to set up accounts which route money to a bank account in Kota with Axis Bank.
Though people on dozens of web forums have recorded their experiences with the scammers, police and trading standards officers in the UK are powerless to stop them.
UK telephone numbers for contacting the company on the sites are not "geographical" tied to a location - but instead allocated to voice-over-internet providers.
That means that the calls connect internationally, but cost the scammers almost nothing when anyone calls them.
In the same way, it costs them virtually nothing to make the calls because the international part of the call goes via the internet.
If the payment has been made on a debit card - as many are- there is no hope of reversing the payment. A number of payment organisations used by the scammers have shut down their accounts. PayPal, the eBay-owned credit transfer company, and AlertPay have both taken rapid action against scam sites which used them.
In March, site hosting company Hostgator shut down one of the longest-running sites used for the alleged scam, F1Compstepuk.com, after complaints.
After confirming with Microsoft that the site was not acting for it, Hostgator immediately shut it down.
But one investigator who has been tracking the growth of the scam says the challenge is that new sites offering the same fake "service" keep popping up "like mushrooms".
At first the scammers tried desperately to maintain the reputation of their sites, by flooding any forum which garnered enough criticism of their activities with postings claiming that the site helped fix their machine.
But the poor spelling and grammar of the replies - allied to internet addresses which show that the commenters are based in India - contrasted sharply with that of people in the UK, the US and Australia complaining about the attempted scam.
Now they have shifted to creating multiple sites from templates, using stock phrases and photos. However, investigators are sure that the same man - and central operation - is behind all of the schemes. Microsoft denies any connection with the companies that call people up offering these services. When contacted about the scams, Microsoft said it was "currently investigating a series of instances in which the business practices of an organisation within the Microsoft Partner Network [that] have given rise to significant concerns from a number of sources. We take matters such as these extremely seriously and will take any action that is appropriate once our investigation is complete."
However, the company has noticed the problem. "Microsoft does not make unsolicited phone calls to help you fix your computer," it says on its website.
"If you receive an unsolicited call from someone claiming to be from Microsoft Tech Support, hang up. We do not make these kinds of calls."