Microsoft is facing a class action suit over a tool that gathers data on users' computers, to detect bootlegged copies of its Windows operating system.

The suit, filed in the US District Court in Seattle on Monday, concerns Microsoft's Windows Genuine Advantage (WGA), an anti-piracy tool the company introduced in July 2005. The WGA program collects hardware and software data, delivering it to Microsoft servers. The stored information is then used to warn of possible piracy violations.

The lawsuit alleges the program violates consumer protection laws in California and Washington state, and laws against spyware, invasive programs that surreptitiously collect data.

In introducing WGA, Microsoft finds computer users already sensitised to the question of how they are notified about software installation and privacy issues. Late last year, Sony provoked controversy by shipping 15 million music CDs containing invasive copy protection software that installed itself on buyers' computers.

Sony eventually compensated users who bought the affected CDs, which had software that installed itself without user consent and transmitted data, after a class-action suit.

The suit against Microsoft contends the company mislead users in delivering WGA to computers, masking it under batches of monthly updates that often include critical security patches. It asks Microsoft to delete all data collected by WGA and provide users with the ability to remove the software from their computers, in addition to damages.

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"In truth and in fact, Microsoft, in its efforts to maximise revenue through anti-piracy measures, mislead consumers and the public as to the true nature, functionality and operation of its WGA," the suit said.

Earlier this month, Microsoft admitted a version of WGA under distribution as a "high priority" update was in fact a test version of the WGA software. In April, Microsoft added a function notifying users if their copy of Windows wasn't valid and causing it to make frequent contact with the company's servers.

In response to user complaints, Microsoft released a new version of WGA this week, allowing people to opt out of notifications.

Users have also complained the software is buggy, labeling copies of Windows as counterfeits when the license may have been legitimately transferred to a different piece of hardware.

For computers suspected of running bogus software, Microsoft has blocked downloads of free tools such as Windows Defender, its antispyware tool, but allowed security patches.

Microsoft officials contacted early Friday did not have an immediate comment.