Microsoft is offering two new licensing options for Windows Vista enterprise customers. The new options are aimed at users who want to take advantage of emerging scenarios for large data centres.

The first gives enterprise users the ability to run the Windows Vista Enterprise client on a diskless computer, said Scott Woodgate, director in the Microsoft Windows product group. The second option, called the Vista Enterprise Centralized Desktop, will allow users run a client version of Windows on servers in a data centre so the OS can run locally via virtual machines.

A diskless PC has no hard drive; instead, the hard drive is stored on the network and an image of the OS is streamed from there into the memory and CPU of the computer, Woodgate said.

Companies interested in protecting sensitive data, such as financial services companies and government customers, have expressed interest in running Vista on diskless PCs for security reasons. However, it is an emerging scenario for data centres, and so for now will only affect a small number of customers, he said. "The target audience is for early adopters," Woodgate said.

To run Windows Vista Enterprise on diskless PCs, customers must use third-party diskless boot software from companies such as Citrix Systems and other Microsoft partners. This software enables the PC to find a copy of Windows on the network and to stream that software back onto the local machine, Woodgate said.

Vista Enterprise Centralized Desktop allows customers to take Windows Vista Enterprise client software and install it on a virtual machine on a server so it can be accessed from a thin or rich client. This licensing scenario is also the first time customers will be able to run Windows client software on servers, Woodgate said.

This option provides a cost-effective option for companies such as brokerage firms where trading brokers view several computer monitors at once - all attached to one PC. This way, a business can run one copy of Windows Vista Enterprise on a server and access the OS in multiple virtual machines, Woodgate said.

The diskless PC option is available now for no extra charge, for customers that already have licensed Windows Vista Enterprise, said Mike Burk, a Microsoft Windows product manager.

The Vista Enterprise Centralized Desktop licence will be available for customers who have Microsoft's Software Assurance subscription service for an additional fee beginning in July. Microsoft is not disclosing how much more customers will pay for the licence as fees for Software Assurance vary per customer, Burk said.