Microsoft has been dragged even further into adding support for a huge enterprise technology trend -- server virtualisation -- in which it has plays little or no part. But on the other hand, it's also restricted the circumstances under which the forthcoming Vista client OS can be virtualised.

Effective immediately, Microsoft has changed the licensing terms of the top-end DataCenter Server 2003 R2 product to include unlimited virtualisation rights. It means that you can run an unlimited number of virtual servers, using whatever virtualised OS you choose, without having to buy more licences.

According to the Redmond giant: "The unlimited virtualisation rights significantly simplify the licensing of Windows Server for large-scale virtualisation, and make it more affordable to consolidate on the Windows Server Platform." Amen to that.

This removes the long-running complaint that Microsoft's licensing terms have been at best ambiguous and at worst too prohibitive, restrictive -- and expensive. It means that VMs, especially those containing Microsoft OS code, can now be moved from server to server at will without worrying about the licensing ramifications.

It's a vital move for those deploying virtualisation technology as, increasingly, virtualisation management systems shuffle virtual servers between and across hardware resources in search of the best performance.

It's a vital move too for Microsoft. The company has very little visibility in the host server virtualisation market, and won't have until its long-anticipated Virtual Server appears in 2008. By then, market leader VMware is expected to have danced off into the distance and moved the game on -- but in the meantime, it pays Microsoft to fit into a game in which where it doesn't own the ball.

What about Vista?

However, reverting to type in a game where the ball belongs almost entirely to Microsoft -- Linux clients notwithstanding -- Redmond has tightened the licensing terms for Vista with respect to virtualisation.

Only two versions of Vista -- Windows Vista Ultimate and Windows Vista Business -- will be capable of running as a VM guest OS.

Product manager Shanen Boettcher said: "Virtualisation is a new technology, and it's primarily used in the business space and by technology enthusiasts. It fits in well with the target audience."

Arguably however, it does seem likely to increase the price of those machines where the OEM chooses to configure machines with VMs for system maintenance applications such as backup, data protection and recovery purposes -- a possibility that's increasingly likely as both Intel's and AMD's processors containing hardware virtualisation assistance start to permeate the market. In such a situation, the OEM will be forced to use a more expensive version of the OS than might otherwise be the case -- or will be deterred by the additional cost.

It's also now clear that Vista licences will be transferable only to one additional machine other than the one with which it was bought. The change made sense, said product manager Shanen Boettcher, because PC lifetimes are getting longer. As a result, the argument runs, most users won't need to migrate an OS licence onto more than one extra computer during the lifetime of Vista version 1.

This doesn't take account of power users and technology enthusiasts, who will change machines far more often. Presumably Microsoft made the change to add differentiation and to reduce piracy. However, one anonymous user said the change may encourage Windows piracy among PC enthusiasts who update their machines on a more regular basis. "Power users will pirate what they need," he said.

Just how this applies to those holding enterprise licences has yet to be clarified but it's likely that such restrictions won't apply.

So it seems like it's business as usual: Microsoft giveth with one hand and taketh away with another -- depending on what it thinks it can get away with.

Additional reporting by Elizabeth Montalbano, Computerworld.