For licensing local desktop virtualization on company-owned PCs, not much has changed since Forrester last looked at the issue in June 2008, Lambert said in her report. If a company subscribes to the Software Assurance program, it can run up to four virtual machines on top of the Windows host operating system—including Vista, XP, 2000 and earlier versions—on a single physical PC. If a company needs five or more virtual desktops, it will need to buy more copies of Windows.

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However, without Software Assurance, a company can't run virtual desktops on top of the PCs.

For all other desktop virtualization scenarios, enterprises will need Microsoft's VECD (Vista Enterprise Centralized Desktop) license. This encompasses initiatives that include running a Windows client OS in a hosted desktop environment—on a server in a data center—or on a PC that isn't owned by the company, such as a contractor's machine or one owned by an employee.

With the VECD, costs vary depending on the device the operating system is running on, Lambert said. If the device is owned by the company and covered by Software Assurance, the cost is $23 per device per year. This includes any system connected to a hosted desktop, which includes users who want to connect to the central hosted desktop not only at work, but also from home.

However, for thin-client devices connected to a hosted desktop, or for PCs that aren't covered by Software assurance, the cost is $110 per device per year. A change in this scenario is that, unlike in the past, the VECD license lets noncorporate PCs—for example, those used by outsourcers and contractors, or those owned by the employee—connect to the hosted desktop environment. That cost is still below the list price of $199.95 for Vista Home Basic and XP Home, and $319.95 for Vista Ultimate.

With the VECD, users can store an unlimited number of virtual desktop systems on physical disks in the data center, which will enable IT staff members to "create, play with and destroy VMs without worrying about complying with their license agreement," Lambert said. "In addition, these VMs can move between servers and storage as needed, allowing for a dynamic environment that caters to user performance needs at any given time." However, the license limits to four the number of virtual desktop machines that can connect to the hosted desktop at a single time.

Overall, the new licensing rules open up options that were hindered under the older licenses. Contractors can now use their own PCs in a company's office. In addition, employees can now bring their own Windows-based PCs to work, a scenario that is gaining in popularity, though it does present headaches for IT administrators over such issues as security.

The new licensing also increases the mobility of a company by enabling employees with corporate PCs to bring their Windows desktop home on a removable media device.

Lambert said for enterprises looking to expand their use of desktop virtualization, the new licensing scheme from Microsoft removes a key hurdle.

"As Windows licensing makes more and more scenarios possible, desktop virtualization is coming to the forefront of computing," Lambert wrote in the report. "While just one year ago, Microsoft handcuffed many organizations that attempted to legally license Windows Client in a virtualized world, they have made steady improvements to pave the way for new computing models—specifically, models that move away from a standard, physical corporate PC."