Microsoft has stepped up its fight against software piracy by requiring users to verify their copies of the Windows operating system.
Around a third of software sold worldwide is counterfeit
Anyone downloading updates for Windows XP will be required to check that their operating system is genuine.
Security updates are exempt from this in order to prevent people spreading viruses across the internet.
A third of software worldwide is fake, according to industry group, the Business Software Alliance.
Microsoft said that more than a third of its software is counterfeit, which it says costs it billions of dollars of lost income.
With Windows accounting for 90% of operating systems sold worldwide, cracking down on the pirates is one of the few ways for Microsoft to expand its business.
"This is another tool in our arsenal," said David Lazar, director of Genuine Windows for Microsoft.
Customers who discover that their copy of Windows is pirated have two options.
They can get a free version of Windows if they fill out a counterfeit report identifying the source of the software, providing proof of purchase and returning the counterfeit CD.
If they are unable to provide all the information, filling out a report will entitle them to receive a copy of Windows XP Home Edition for the reduced price of £56 or Windows XP Professional Edition for £86.
The new policy is part of Windows Genuine Advantage, a scheme set up by Microsoft in September to encourage people to use legitimate software.
The firm has been offering incentives, in the form of freebies such as photo slide-show software, to persuade people to use the service.
"We want to give customers a reason to purchase the genuine product," said Alex Hilton, anti-piracy manager for Microsoft.
So far, 45 million copies of Windows have been verified via the system, although Microsoft has, as yet, no figures to gauge what proportion of these failed the test.
Neither has it set a figure on the level it would like to see piracy reduced to.
"Aspirationally we want to remove piracy altogether and we feel the majority of people want to use our software legitmately," said Mr Hilton.
He advised anyone looking to purchase Windows XP to make sure they only do so via trusted shops and websites.