clipped from www.nytimes.com

In the past, said Mike Neil, general manager of virtualization strategy at Microsoft, customers paid a license fee when Windows was installed on a physical machine. But he said virtual-machine software breaks the tight link between the operating system and the hardware, raising the possibility that customers could be using Windows more but not paying for it. So now, he said, the license fee is based on when a copy of Windows is used, whether in a virtual or physical machine.