Microsoft will limit the number of machines to which users can transfer Windows Vista licenses, the software giant has revealed.

When the new OS arrives early next year, consumers will only be able to transfer the licence they purchase to one other machine (including the one they originally purchase with Vista), said Shanen Boettcher, Windows general manager at Microsoft.

The change makes sense because "lifetimes for PCs are getting longer", he argued. Most likely, a user will not need to transfer an OS license to more than one computer before a new OS is available. "It's a fit for what most customers do," Boettcher said. There is no limit on the number of PCs Windows XP can be moved on to.

Another change Microsoft has made to Windows client licensing since XP is that only two versions of Vista - Windows Vista Ultimate and Windows Vista Business - can be run as a virtual OS in a virtual machine. "Virtualization 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 also has put more restrictions on anti-piracy features in Vista, and users who do not verify that their version of the OS is authentic with a software license key within 30 days of its activation will lose the use of certain features of the PC until they do so.