Microsoft is taking another step to crack down on software piracy this week. On Tuesday, customers who want to update Windows or download other popular software from Microsoft.com will first need to install counterfeit-sniffing code on their PCs.

The measures are part of a change to Microsoft's Windows Genuine Advantage program, which the world's largest software maker has been testing since last September. Users whose PCs pass muster as running legitimate copies of Windows will be able to download software updates to the operating system and other Microsoft products, as well as additional software Microsoft makes available online, such as the Windows Media Player, DirectX for games, an anti-spyware tool, and new photo-sharing software.

Before customers are allowed access to Microsoft's Update or Download Center Web sites, they will have to install a small program that looks for signatures of pirated software, such as a product-identification code that Microsoft knows to be leaked, says Microsoft director David Lazar. Previously, users of those sites had the option whether to undergo the scan. Microsoft will continue to make security updates available to all Windows users, however.

Counterfeit software "is a monumental problem for Microsoft," says Bonnie McNaughton, a senior attorney at the company. Microsoft last week reported that Windows generated $12.23 billion in revenue and $9.44 billion in operating income during the fiscal year ended June 30. Overall, Microsoft reported revenue of $39.79 billion for the year.

A study by the Business Software Alliance and market researcher IDC released this spring estimates that 35% of PCs worldwide ran pirated software last year. At a meeting with Wall Street analysts a year ago, Microsoft executives estimated that 22% of PCs in the United States--about 12 million machines--ran pirated software. In China, 13 million PCs did, Microsoft estimated.

The official launch of Windows Genuine Advantage is part of a broader crackdown on software pirates by Microsoft over the past two years, McNaughton says. In June, Microsoft filed four lawsuits against Internet resellers the company says sold pirated copies of its products. Microsoft has received hundreds of requests from its legitimate resellers to cut down on dealers of counterfeit products, she says.

During the Windows Genuine Advantage pilot, 47 million users signed up to have the counterfeit-scanning ActiveX control installed on their machines, out of 82 million users who viewed the option online. Users will no longer be required to enter their Windows product key number as part of the anti-piracy test, however. Microsoft says that simplifies the process. Users with PCs that contain counterfeit software may still qualify for free or discounted legitimate copies of Windows, according to Lazar. He adds that as part of its PC-diagnosis mechanism, Microsoft won't collect any information that could identify individual computer users, or allow the company to contact them.