In a decision that highlights Windows Vista's hefty system requirements, Microsoft said Thursday that it would allow computer makers to continue to sell the older Windows XP operating system on "ultra low-cost PCs" for an extended period.

Microsoft said it would allow system vendors to preload the Home edition of Windows XP on ULCPCs through June 2010, or one year after the next version of Windows becomes generally available.

Microsoft defines ULCPCs as, among other things, systems that use discount-line processors and lack a separate graphics card. An example of such as system is the Asus Eee PC, which runs Windows XP or Linux and sells for less than $400.

Such low-spec machines would be incapable of running Vista.

To experience all of Vista's features, PC users need a computer with at least a 1-GHz processor, 1 GB of memory, and a 40-GB hard drive. By contrast, Windows XP Professional requires only a 300-MHz processor, 128 MB of RAM, and a 1.5-Gbyte disk.

Without continued access to XP, vendors like Asus would be forced to offer only Linux on their systems. It's a situation Microsoft is trying to avoid, particularly as sales of ULCPCs rise in emerging markets like India and China.

Microsoft is terminating Windows XP's shelf life for most PC makers on June 30, though independent system builders will have access to the OS through January 2009. Microsoft has said it expects XP sales to account for as little as 15% of its operating system revenues in its current fiscal year, which runs through June.

Microsoft introduced Windows XP in late 2001. The company ordinarily makes operating systems available only for four years after launch date. But delays in producing Windows Vista, which debuted in January of 2007, forced Microsoft to continue selling XP longer than planned.

Microsoft had originally planned to shelve Windows XP on January 30th.