Microsoft is limiting the number of machines to which users can transfer Windows Vista licences as part of licensing changes for its new Windows client OS (operating system).

Consumers will be able to transfer the Vista licence they purchase to only one machine other than the one for which they originally buy the OS, said Shanen Boettcher, Windows general manager at Microsoft.

He said the change made sense because lifetimes for PCs are getting longer. Most likely, a user will not need to transfer an OS licence to more than one computer during the time the OS is the latest one available, Boettcher said.

"It's a fit for what most customers do," he said. Boettcher added that XP did not have a specific limit for the number of times the licence was transferable, but that Microsoft wanted to be specific about transfer rights with Vista.

Another change Microsoft made to Windows client licensing since XP is that only two versions of the new OS – Vista Ultimate and Vista Business – can be run as a virtual OS in a virtual machine.

"Virtualisation is a new technology, and it's primarily used in the business space and by technology enthusiasts," Boettcher said. "It fits in well with the target audience for these [Windows versions]." Windows XP did not have any restrictions on running in a virtual environment, he added.

Microsoft has also tightened up antipiracy features in Vista, and users who do not verify that their version of the OS is authentic with a software licence key within 30 days of its activation will lose the use of certain features of the PC until they do so. The company unveiled this feature, called its Software Protection Platform, about two weeks ago.