Microsoft expanded its "Windows Genuine Advantage" program, requiring users of its software to verify their copies of software in order to receive add-ons to Windows XP.

Security-related updates to its software, which are used to plug software flaws exploited by viruses and hackers, will be exempt so that all users can avoid infection and, in turn, spreading viruses across the Internet, said David Lazar, director of Microsoft's Windows Geniune program.

"This is another tool in our arsenal," said Lazar, adding that a third of installed Windows programs worldwide are pirated copies.

Microsoft initially began the program on a trial basis last fall, and expanded it to more than 20 countries in February. Microsoft is targeting software piracy in places such as China, Norway and the Czech Republic, where the use of pirated software is more widespread.

Microsoft said it was trying to reclaim revenue that the company and its reseller lose due to piracy, which it said resulted in billions of dollars in lost income.

Microsoft said, however, that it would no longer require legitimate Windows users to enter a product code, a string of 25 numbers and letters, in order to verify that a copy of Windows is legitimate. Instead, Microsoft said it will check hardware data to verify that multiple copies of Windows aren't being installed on different personal computers.

Customers whose copies of Windows are found to be pirated have two options: send in a pirated CD and fill out a piracy form to receive a legitimate copy for no charge; or, in the case that there is no CD or proof of purchase, buy a new copy of Windows XP Home for $99 or Windows XP Professional for $149.