Microsoft has sent a letter to customers saying that it will extend critical support for Windows XP until 2014, three years longer than is usual.

The company also pledged that its next operating system, Windows 7, will be available by 2010.

In the meantime OEMs will be able to supply systems with Windows XP preloaded as a 'downgrade' until 31 January 2009.

Given that Microsoft usually supports operating systems for only 10 years after their launch, the move can be seen as an admission that it is expecting some businesses not to bother upgrading to Vista.

"Windows Vista is a very significant step forward, but our customers have made it clear that they want broader support for devices and applications in order to enjoy the overall experience," says the letter from Bill Veghte, senior vice president of the Microsoft Windows Business Group.

"During the last year, we have worked diligently with our hardware and software partners to improve compatibility to remove the barriers that prevent users from taking advantage of the important advancements Windows Vista delivers. It has been a year of exciting and critical progress."

The letter also promises that Windows 7 will be released "approximately three years after the January 2007" release date for Vista.

Veghte acknowledges that there have been compatibility problems between Vista and peripherals manufacturers, and promised that lessons have been learned.

"You've also let us know you don't want to face the kinds of incompatibility challenges with the next version of Windows you might have experienced early with Windows Vista," he wrote.

"As a result, our approach with Windows 7 is to build off the same core architecture as Vista so the investments you and our partners have made in Vista will continue to pay off with Windows 7.

"Our goal is to ensure that the migration process from Vista to Windows 7 is straightforward."