In the latest setback for Microsoft's Windows Vista operating system, Dell plans to continue offering the older Windows XP on business systems until 2010, InformationWeek has learned.

Dell will include Windows XP Professional as a pre-installed option on its OptiPlex desktops and Precision workstations for at least the next 20 months, according to a Dell employee with knowledge of the computer maker's plans.

Microsoft has said it would officially stop making Windows XP available to large PC vendors on June 30.

Dell's plan could conceivably allow its business customers to skip Vista, the current version of Windows, altogether. Microsoft has said it plans to launch Vista successor Windows 7 in 2010.

Buyers who want a Dell system with XP pre-installed after June 30 will technically be purchasing a system with a license for Vista. However, Dell will also include a preinstalled copy of XP on systems marketed as Windows Vista "Bonus" edition.

The "Bonus" option will be available on Dell business systems that can be configured with Windows Vista Business or Windows Vista Ultimate. To downgrade those systems to XP, users will need to call Microsoft to receive an XP activation code.

Microsoft has little choice but to provide the codes.

Its OS license agreement allows users to downgrade a purchased operating system to the previous version at no additional charge. Dell is simply making it easier for its customers to exercise that option by pre-installing XP long after its official expiration.

The decision by Dell, the nation's second largest computer maker, to pull an end-run around Microsoft's Windows XP timetable is the latest example of the industry's lukewarm embrace of Windows Vista.

Implicit in Dell's move is a belief that a considerable portion of its business customers are not willing to upgrade to Vista, despite the fact that the OS has now been available for more than a year and has been fully updated with Service Pack 1.

Many business users have complained about Vista's resource requirements, intrusive security features, and lack of compatibility with older systems and applications.

What's not immediately clear is from where Dell, famous for operating on razor thin inventories to control costs, will continue to obtain Windows XP media after Microsoft's cut-off date. It could be relying on existing stock or stamping its own with explicit permission, or perhaps a wink and a nod, from Redmond.

It might also be using back channels to obtain copies of XP. Microsoft has said it would make the operating system available to small manufacturers of low-cost systems incapable of running Vista until 2010. "Independent" system builders can also get XP until 2009, according to Microsoft.

A Microsoft spokesman commented: "It's standard practice for original equipment manufacturers, retailers, and system builders to continue offering the previous version of Windows for a certain period of time after a new version is released. Dell is exercising their right to offer Windows XP as an option, though we expect the majority of their customers to continue to order the latest and greatest technology, including Windows Vista."