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Microsoft is to withdraw an anti-piracy tool from Windows Vista, which disables the operating system when invoked, following customer complaints.

The so-called "kill switch" is designed to prevent users with illegal copies of Vista from using certain features.

But the tool has suffered from glitches since it was introduced with many Windows users claiming that legal copies of Vista had been disabled.

Microsoft says its efforts have seen a drop in piracy of its software.

In a statement released by the company, Microsoft corporate vice president Mike Sievert, said: "Users whose systems are identified as counterfeit will be presented with clear and recurring notices about the status of their system and how to get genuine copies."

'Take action'

"They won't lose access to functionality or features, but it will be very clear to them that their copy of Window Vista is not genuine and they need to take action."

Microsoft has described the new approach as a "change of tactics". It said efforts to tackle piracy had seen numbers of fake copies of Vista at half the level of XP, the previous Windows operating system.

The change will take effect with the release of Service Pack 1, a major update to Windows Vista.

Customers who buy a copy of Windows Vista or have the operating system (OS) installed when they buy a new PC are required to validate the OS with Microsoft.

An online tool, called Windows Genuine Advantage, checks the authenticity of the OS to determine if it was legally acquired.

The tool can "lock" Vista from further use if it believes it is an unauthorised copy. But many users have complained that the system is not working because legally bought copies result in error messages.

It was introduced in 2006 as voluntary option, but became mandatory with the release of Vista, and had problems from the day it was introduced. Mr Sievert added: "It's worth re-emphasising that our fundamental strategy has not changed.

"All copies of Windows Vista still require activation and the system will continue to validate from time to time to verify that systems are activated properly."

Microsoft said it had pursued legal action against more than 1,000 dealers of counterfeit Microsoft products in the last year and taken down more than 50,000 "illegal and improper" online software auctions.