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.

What's made hosted desktops possible is two things - broadband and webmail, argues Extrasys technical director and founder Jay Korde.

"People hosting their email has helped - consumer choices are now influencing business choices too," he says. "It's heading for a case where technology will just be assumed, it'll be a service like water or electricity, and like the electrical grid, you take what you need. Services that sound clever now will just be assumed in four years time."

Extrasys doesn't just virtualise desktops, it also virtualises applications. Not only does this mean you can run an application without installing it on your PC, but you can even run apps side by side that would normally be incompatible, such as different releases of Microsoft Office.

"Bigger customers take single applications to run on the desktop, for example they might need an app for 12 months to share data, without having to acquire the IT to run it," Korde says. "It's good for distributed project teams - you get access to shared applications and files."

He adds that Extrasys makes a lot of use of application virtualisation technology, in particular Softricity, which is now owned by Microsoft and allows apps to be packaged and streamed out as needed.

"Virtualising the application is a big part of what we do," he says. "The idea of application streaming came from a request to Softricity to make a gaming application run through firewalls - our clients need that same ability.

"A lot of core Microsoft applications are good at being hosted without streaming. It's older applications that compete for resources, or weren't even written to run over a LAN, that are the problem. We can run older apps too because of our wrappers and extra layers, but it's also about not being prescriptive."

Another advantage of virtual PCs is that you don't need a PC to run them, says Sachin Duggal, the founder and CEO of hosted desktop pioneer Nivio, which is based in Europe and India.

Nivio recently signed up to support AMD's 50 by 15 Initiative, which aims to bring Internet and IT access to 50 percent of the world's population by 2015. Duggal argues that hosted virtual desktops will be key to achieving that ambition, because so many of that 50 percent simply cannot afford their own PCs.

"For example, in Indian cities you have broadband, but not enough devices to use it," he says. "So at the moment the ISPs are all fighting for the same customers - people with PCs.

"Our approach means that any device - even a Linux PC or a games console - will give you a single universal desktop. And you can rent applications too. But the key thing is it's a full desktop, the full Windows XP experience, because that's what 96 percent of people expect - right now there's 45-odd web OSes, but they're not Windows and they need a fat client."

Accessibility anywhere is also key, he says, because while people can get connected to the Internet one way or another, they often don't control the device that they connect through.

"50 percent of Indian students use Internet cafes, and 90 percent of cafes block USB sticks," he explains. So having your own portable PC set-up - on a bootable USB stick, say - isn't feasible.

"We have seen a lot of people use Nivio for backup too, because it means their content is always there, or to give access for their kids," he adds. "Small businesses use it to rent software or enable mobility, and there are a lot of students, especially in BRIC [Brazil, Russia, India, China].

"We're also working with AMD to superimpose a new operating system-independent user interface. It just needs a browser and Java or ActiveX, the aim is for it to be as easy to use as a mobile phone, yet you can still click out of it to a standard XP desktop."

Giuseppe Amato, AMD's European technical sales and marketing director, adds that far from trying to block the use of bandwidth-intensive applications such as hosted virtual PCs, he thinks ISPs will welcome them.

"In some countries, ISPs such as Telecom Italia have had to give you a PC to get you to buy broadband, but now they don't," he says.

"In a few years we will be always-connected. With WiMAX, ADSL, UMTS and so on, it will no longer be a dream, and the bandwidth limitation will be removed."