Microsoft will start to give some customers the benefit of the doubt with an updated version of its software antipiracy tool, Windows Genuine Advantage Notifications.

The company has added a new category to the tool, "indeterminate," for instances where it can't be sure whether the OS running on a user's PC is legitimate or not, said Michala Alexander, Microsoft's head of antipiracy for the U.K.

Previous versions of the software labelled an OS as either valid or invalid. In the latter case it would block the customer from downloading software updates, except for critical updates such as security patches.

However, some users have complained that the Notifications tool incorrectly identified their copy of Windows as unlicensed. The new category aims to address that problem, in some cases not ruling one way or the other on whether a user's software is pirated.

The issue has arisen partly because, through no fault of the customer, the wrong product activation key can sometimes be used to unlock a legitimate copy of the OS.

For example, a PC repair shop might reinstall an OS on a customer's machine with a product key that it uses over and over, Alexander said. She said she saw one repair shop that had a product key written up on a whiteboard so it could be reused easily. Software pirates can also complicate matters, by generating fake product activation keys to try to unlock software.

The Notifications tool checks the activation key on a customer's PC and compares it to a database of legitimate keys. A computer may pass the test at one time, but six months later it may fail it if other computers have subsequently been registered using the same product key, Alexander said.

The situation makes it difficult sometimes for Microsoft to make a clear call. "The last thing we need are false positives," she said.

When a customer's software is flagged as "indeterminate," they will be redirected to Microsoft's Web site for more information. One way out of the problem would be to re-enter a legitimate product key, which does not require the user to reinstall the OS, Alexander said.

Microsoft will continue to update Notifications every three to four months, she said. Earlier this month it released an update through Windows XP's Automatic Updates. It also rolled out 21 localized versions of the program for countries including China, Portugal, Russia, and Korea.

Complaints from users have prompted Microsoft to make several changes to Notifications since it debuted in July 2005. It dropped the persistent pop-up messages that used to warn people their software might be unlicensed, for example.

Notifications is one prong of Microsoft's antipiracy strategy, which also involves suing counterfeiters and visiting PC stores to ensure that sellers are installing the OS properly.