Microsoft said supposedly pirated copies of its new Vista computer operating system "will be of limited value" to those who use them. Microsoft responded Tuesday to reports that some Web sites have been circulating pirated copies of Vista and the Office 2007 applications suite.

The pirated Vista comes with a product key that users can enter to activate a version of the products on their computers without paying for them, according to a report on the Web site of The Sydney (Australia) Morning Herald newspaper. A product key is a unique serial number tied to each package of a software product.

A second download, called an "activation crack," can then be applied that bypasses the activation process intended to guarantee that the Vista OS being downloaded is legitimate, the Herald reported. Pirated copies of Office 2007 can be downloaded just with the product key with no second activation code required. But Microsoft said in a prepared statement that those pirated copies of the OS won't work for long.