Microsoft's anti-piracy tool has marked more than one in every five copies of Windows as bogus, the Redmond, Wash., developer said Tuesday, while more than half a million users may have been mistakenly pegged as pirates.

As it beats the drum about the danger of pirated software, particularly Windows, prior to the release of Vista, Microsoft released some figures from its Windows Genuine Advantage program. WGA uses software downloaded to the PC to authenticate Windows XP before allowing a user to retrieve automatic security updates and other software.

In the 30 months since WGA's 2005 launch, 512 million have tried to validate their copy of Windows, Microsoft said. The "non-genuine" rate, or the fraction pegged as counterfeit, stands at 22.3%. In other words, 114 million users who ran the audit software were labeled pirates by Microsoft.

"That's actually lower than the piracy rate overall," explains David Lazar, the director of Microsoft's Genuine Windows. Research done by the Business Software Alliance, an industry group dedicated to combating piracy, puts the global piracy rate at 35%.

But of the 114 million told they were running phony Windows, only 56,000 filed a counterfeit report, a requirement before Microsoft will issue a free or reduced-price copy of the operating system. "The report is what customers fill out when they feel they've been victimized by another party," says Lazar, who declined to specify the number of free copies of Windows the company has handed out to duped users in the last two-and-a-half years.

"That [56,000] is a very good number, and gives us a huge amount of data," Lazar says. Microsoft uses the information in the reports to spot dodgy system builders or patterns in piracy.

WGA has been criticized by users and some analysts for frequently getting the real versus counterfeit question wrong, mistakenly identifying valid copies as illegitimate. Lazar acknowledged that this "false positive" rate was a burden to users and Microsoft.

"Every single [false positive] is a problem for the customer and for us," he says.

Microsoft has been tight-lipped about the false positive numbers, but Tuesday Lazar said it was "less than half a percent." At that rate, as many as 571,000 people have been incorrectly identified as pirates.

"Our daily goal is to bring that as low as possible," says Lazar. "Any customer who feels that they've been given bad information or are a victim of a false positive has every right to follow up [with us] to find out the reason for the failure. In the majority of cases, we do find the reason for the failure."

One recent enhancement meant to trim the number of false positives, says Lazar, was a November update to the WGA Notifications software, a tool that plants messages on the screen to mark Windows as fake. The update changed how unresolved errors were tabulated. Rather than brand systems that generated validation errors as counterfeit, they are now categorized as "unable to complete validation," says Lazar. Users of those PCs do not see the nag notices.

Windows Vista, which rolls out to retail next Tuesday, has its own anti-piracy tools that differ substantially from Windows XP's. Vista, for example, renders the operating system almost inoperable -- only the Internet Explorer browser works -- if it suspects the copy is bogus.

Editor's Note: This story was changed on Jan. 24 to correct the number of false positives attributed to the WGA software.