Microsoft has released an update to Windows Vista that will shut down unauthorised versions of the OS that allow users to skip the product's activation system.

The move comes as pirated copies of Vista are already making the rounds, just two weeks after the product was released to business customers.

The update, which Microsoft has dubbed "frankenbuild", detects tampering of Windows Vista code that would allow users of the OS to work around the product's built-in activation system, which requires users to validate their copy of Vista with a product activation key to use the full version of the product after 30 days.

Frankenbuild mixes files from various test and final versions of the software. It will require only systems in which it detects specific tampering to go through a validation check for authenticity, according to a posting on the Windows Genuine Advantage blog.

If a version of Vista that has used a workaround to avoid product activation is detected, a user of that software will have 30 days before the OS goes into a reduced functionality mode, Microsoft said. In this mode, all users can do is access their existing files and surf the web for an hour before having to log back on to the software.

The pirating of Windows has been a perennial problem for Microsoft, particularly in developing countries. The company began coming down hard against piracy last year with the widely criticised Windows Genuine Advantage system, which initially required users to validate their copies of Windows if they wanted to use Microsoft's update services.

Microsoft took its antipiracy campaign one step further with Vista by building the validation system directly into the OS. This system requires a Windows Vista user to validate the software through a product activation key within 30 days of using the OS to avoid having the software go into reduced functionality mode.