Microsoft has for a second time extended the period in which customers who purchase a Windows 7 PC can "downgrade" the operating system to Windows XP.

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Under the latest extension, Windows 7 Professional or Windows 7 Ultimate users can downgrade to a similar version of XP or, in a far less likely scenario, Vista, through the entire Windows 7 lifecycle—which runs through 2020.

Windows 7-to-XP downgrade rights were most recently set to expire within 18 months of Windows 7's October 2009 release or with the arrival of Windows 7 SP1, depending on which came first. Previously, Microsoft said the downgrade option would be available only for the six months following Windows 7's debut.

The downgrades, while open to all owners of the higher end versions of Windows 7, are aimed primarily at corporate buyers—particularly those at small and midsize companies that don't subscribe to one of Microsoft's subscription plans. Businesses have been slow to move off Windows XP mostly due to concerns that their applications won't be compatible with the newer Microsoft operating systems.

In a seemingly logic-defying statement, a Microsoft spokesman insisted the XP downgrade extension was necessitated by Windows 7's growing popularity.

"To support our customers 'unprecedented move' to migrate their PC environments to Windows 7, we have decided to extend the downgrade rights to Windows XP Professional beyond the previously planned end date at Windows 7 SP1," said Brandon LeBlanc, Microsoft's in-house blogger, in a post Monday.

LeBlanc insisted the move will make it easier for IT pros to manage their fleets of corporate PCs.

"While the majority of customers are actively transitioning to Windows 7, and PC manufacturers are focused on delivering PCs and devices with Windows 7 preinstalled, our business customers have told us that removing end-user downgrade rights to Windows XP Professional could be confusing, given the rights change would be made for new PCs preinstalled with Windows 7," LeBlanc wrote.

"And managing a hybrid environment with PCs that have different end-user rights based on date of purchase would be challenging to track," he added.

LeBlanc said PC makers such as Dell and Hewlett-Packard will no longer be allowed to offer Windows XP as a preinstalled option on new computers after October 22nd, which marks the 1-year anniversary of Windows 7's debut. Users wishing to downgrade after that date will presumably have to take the DYI approach after obtaining XP media from Microsoft or their OEM.