In a move that has caused an unexpected stir in China, Microsoft Corp. went on the offensive against users of pirated software this week, sending millions of computer users a software update that can turn their desktop wallpaper black.

The security measure, part of a world-wide antipiracy initiative that began in August, was sent out through a software update that users of the Windows XP operating system would have been prompted to download.

The update wasn't limited to China and doesn't affect a personal computer's functionality, but it has prompted vocal protests from users on the Internet here. Some people called it a violation of their rights, even though many may not have paid for their Microsoft software.

The move is perhaps the most attention-grabbing yet by the Redmond, Wash., software giant, which has long attempted to discourage software piracy. Piracy has been a particularly stubborn problem in China, the world's second-largest personal computer market by units sold.

Microsoft also lowered the price of its software this month in China in a temporarypromotion and now charges less than $30 for home and student versions of Microsoft Office, down from $102.

The dramatic wallpaper change can be reversed, and it doesn't render computers unusable. But if it detects that an illegitimate version of Windows is running, it will interrupt users periodically with notifications that they may be victims of software counterfeiting. Volunteers are already circulating programs on the Web that users can download to get around Microsoft's add-on.

The program is 'part of Microsoft's ongoing effort to protect customers and partners from counterfeit software and to increase customer awareness of the value of genuine Microsoft software,' a company spokesman said. 'This is a completely voluntary program' because users need to actively download the add-on in order to participate.

But Tuesday, when it was launched, PC users in China lashed out at the program. 'We do not stand up for piracy, but against your company for not thinking how the users feel,' wrote one blogger, called Ling Ge, in an open letter to Microsoft.

Microsoft responded to the complaints Wednesday, saying the measure was meant to help those who are duped into buying illegal software at full price, a user-segment that Microsoft estimates to account for 20% of all users of pirated software. The company also said the software would not affect users' applications nor cause any data losses.

China is increasingly important to technology companies worldwide, especially as technology budgets shrink in the wake of the economic downturn. Microsoft, which has been expanding its staff rapidly here, struggled until recently because of rampant software piracy. But in the past few years, the company has joined with personal-computer makers, including Hewlett-Packard Co., Dell Inc. and Lenovo Group Ltd., to install Microsoft applications on computers before sale, which has helped its business.

In addition, as part of its enforcementstrategy, Microsoft has been filing lawsuits against companies using pirated versions of its software. Even when it wins, however, software developers say China's piracy problems are far from solved, partially because intellectual property rights violations aren't treated as criminal offenses in China.

Piracy can take place a number of ways in China both knowingly and by accident, where users can easily pirate software for less than two dollars or download it for free. Many buy their computers from markets, where sellers can easily pre-installpirated software in order to sell PCs at lower prices without the user knowing.

This was true for Chen Xuemei , a 33-year-old from the western city Chongqing whose desktop turned black Tuesday. 'When I bought [my computer] back from the shop, it already had everything installed by the staff there,' she said. When her screen turned black, she 'had no idea what was happening and had to call the technician in my company to help.'

Loretta Chao / Juliet Ye