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And once computers loaded with XP have been cleared from the inventories of PC makers such as Dell and HP, those consumers who want to keep using the old operating system will have to buy Vista and then legally "downgrade" to XP.

Microsoft says it will still allow smaller PC shops to buy XP for resale through the end of January 2009 and a version of XP will also remain available for ultra-low-cost PCs such as the Asus Eee PC.

Some computer users rallied around a "Save XP" petition had been clamoring for Microsoft to keep selling XP until its next operating system, Windows 7, is available, which Microsoft said it expects to release "sometime in 2009".

However, in something of a reprieve for XP-philes, last week, Microsoft said it would extend technical support for six-year-old Windows XP through 2014, instead of 2009 as planned.