The idea of every worker having their own PC hardware and software will come to be seen as one of the great follies of computing - or so the proponents of virtual desktops would have us believe.

And it must be said that they do have a point. Desktop systems need support, applications must be bought and installed, and of course the ever-advancing pace of technology means that users come to expect regular upgrades too.

Far better, they say, to bring it all back to the centre. Sure, give each user their own customisable desktop, but put the hardware and software where it can be managed, and where you can get economies of scale, and let the users gate in - just as they do already to their Hotmail and Googlemail accounts.

The first big proponent of this concept was Citrix with Metaframe, which was followed by Microsoft's Windows Terminal Services, and since then the options have continued to multiply. You can even run the basic desktop locally and then stream your applications out from the data centre in virtual form.

The market for desktop virtualisation software of this kind will be nearly $2 billion by 2011, predicts IDC analyst John Humphreys. "We feel virtual machines for desktop computing is one of the most exciting developments within the technology industry in recent years," he says.

"We see significant opportunity for organisations to improve the efficiency with which they provide computing resources through virtualised client computing technologies. They not only provide a familiar user experience, but also help centralise desktops and improve data security and user productivity."

Targeting the enterprise desktop

Perhaps the biggest push at the moment is behind virtual desktop infrastructure, or VDI. In this scheme, the desktops run on a virtualised server and are accessed using thin clients or PCs running thin client software. For companies such as Citrix and VMware, VDI is a big opportunity.

"The desktop is fundamentally changing," claims Jerry Chen, senior director of enterprise desktop solutions at VMware. "Our customers are transforming the way they manage their desktop infrastructure, and replacing traditional PCs with centralised desktops that can be more effectively managed and controlled.

"Users of virtual desktops can enjoy reliability, data protection and disaster recovery capabilities that have traditionally been available only for server applications. In addition, they get the flexibility of being able to access their desktops from many locations and devices."

Virtual PCs can also be very relevant for organisations with many user locations to support, according to Chris Bailey, the MD of hosted desktop specialist Extrasys, as they mean there's a lot less on each site that requires local management.

"The education market is proving interesting, especially schools, both for applications such as SharePoint and for full desktops," he says. "It saves them having to buy and run their own servers."

That's certainly true in Collier County, Florida, although the district school board there chose to run its own servers rather than use hosted ones, says its director of technology Tom Petry.

"We are in the process of virtualising 21,000 desktops using VMware VDI," he explains. "We have seen first-hand the manageability and control it brings, including the security and business continuity benefits of centralising our desktop infrastructure into datacentres - no small thing in hurricane country."

There are issues with VDI, however. It requires both server load balancing and connection brokering technologies, the latter to provide security and ensure that users are connected to the right desktop. And some experts have criticised it for being resource-heavy - the server's resources are shared among the virtual PCs it hosts, so each can support only a few users.

But while Metaframe and WTS may support more users per server, not all applications will run on them unmodified, whereas a virtual PC will run just about anything.

Plus, with virtual PCs you don't even have to run the server yourself, you can instead choose to have it hosted for you. There's also more than one way to skin the cat, with the Virtuozzo style of virtualisation economising on resources by providing each user with their own desktop, but on the same shared core OS.