Windows Vista may have new features for mobile computers, but the operating system itself is becoming considerably less portable.

Under changes to Microsoft's licensing terms, buyers of retail copies of Vista will only be able to transfer their software to a new machine once. If they want to move their software a second time, they will have to buy a new copy of the operating system. In the past, those who bought a retail copy of Windows needed to uninstall it from any machine before moving it to another machine, but there was no limit to how many times this could be done.

"It was something that had been abused from a piracy perspective before," said Shanen Boettcher, a general manager in Microsoft's Windows Vista unit. "We're just being clear it's one move from machine to machine that you are licensed for."

The software company will use its antipiracy programs, including its recently announced Software Protection Platform, to enforce the new changes, Boettcher said.

Separate rules apply for the versions of Windows installed on new PCs, which is how most people get their copy of the software, Boettcher said. In most cases, copies of Windows purchased on a new PC cannot legally be transferred.

The licence changes also apply to virtualisation, in which a computer runs multiple operating systems, or multiple copies of the same operating system, at the same time. Customers can only transfer the copy of Windows once, including a transfer from one physical machine to a virtual machine, or from a virtual machine on one PC to a virtual machine on another PC.

"Virtualisation is a new technology," Boettcher said. "We are going to learn more about the use cases as we move forward."

People who have specific questions can call customer support, he said.

Microsoft is also making some other changes as far as virtualisation goes. Although any Windows version can serve as the primary, or host, operating system, only the Business and Ultimate versions of Vista can run as guest operating systems in virtualisation. In Windows XP, each virtual instance of the OS required a separate licence, but there were no restrictions on which versions could act as guests.

Large businesses that obtain Windows Vista Enterprise edition through a volume-licensing contract can run up to four instances of Vista on the same machine with a single licence. Developers in Microsoft's MSDN program can also use more copies of the operating system for testing purposes, Boettcher said.

The change is significant for technology enthusiasts, as well as for Mac users running software, such as Parallels Workstation, that allows Windows to run at the same time as the Mac OS.

The rule change would not have an impact on Apple Computer's Boot Camp software, which installs Windows in a separate partition and allows users to run the Mac OS and Windows, but not at the same time.

Also as part of the changes, Microsoft extended the warranty for Windows. Buyers of retail copies of Vista will get a one-year warranty, which is typical of most Microsoft software, as opposed to the 90-day warranty that comes with XP.

Boettcher said that Microsoft has heard some concerns regarding virtual machine issues, but doesn't think the licence changes represent a threat to Vista sales. "It hasn't come up as any kind of a blocker for adoption," he said.