Windows Vista 's licensing restrictions are as tight as a drum, but Microsoft is to alter some clauses to enable different types of PCs to be tried out. Software licences usually assume one thing - that the whole of the PC will be in a single physical location. And that's where Microsoft is to change things, by taking into account that a single PC can often act as many different machines through virtualisation technology.

Equally, a client machine in one location may use storage in another and so pull its operating system from a different physical location. The Vista licensing provisions for these so-called 'diskless' PCs will also stretch to PCs where processing or graphical power is handled in a different location.

"We think of a diskless PC as simply a PC that runs Windows but does not store Windows or data locally because it does not have a hard drive," says Scott Woodgate, Director of the Windows Business Group at Microsoft.

"The customers that have been requesting diskless PCs tell us they want the ability to move their Windows data and applications to centralised storage hardware such as a Storage Area Network (SAN), while still maintaining the local computing characteristics and experience of a PC."

The change comes in the face of competition from other virtualisation technologies such as that from VMware. The company's software enables customers to deploy hundreds of desktop installations from a centralised location.

The new Vista version - Windows Vista Enterprise Centralized Desktops - is only available to Microsoft partners under the Software Assurance programme. The idea is that Microsoft's key associates can have more choice in how they decide to deploy Vista.