The pay-as-you-go plan is the most recent move by Microsoft in a series that has repeatedly lifted restrictions once put on the 7-year-old Windows XP.

In early October, for example, to the availability of Windows XP for larger computer makers, dubbed OEMS, for "original equipment manufacturers." Rather than cut off OEMs, such as and Hewlett-Packard Co., as of Jan. 31, Microsoft shifted the deadline for obtaining Windows XP Professional media to July 31 of next year.

OEMs include Windows XP discs with new PCs that they had "downgraded" from at customer request. The end-around XP's retail sales retirement date -- June 30, 2008 -- is a popular means for users to purchase new systems with the older OS preinstalled. By some estimates, are downgraded from Vista to XP.

Dell, for instance, has been selling downgraded PCs at a above the cost of equipping the machine with Vista Home Premium, the most popular, but not the priciest, version of the newer operating system.

Earlier this year, Microsoft had loosened the XP rules to allow makers of low-cost notebooks, and later budget-priced desktops, to sell machines with until June 30, 2010.