June 30 of this year and Jan. 31, 2009, may be the deadlines for U.S. retailers and custom system builders, respectively, to sell Windows XP. But, due to exceptions Microsoft has made, the operating system will be available for at least the next two years for those who purchased business and Ultimate versions of Vista, as well as for customers in certain geographies.

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Microsoft has made it widely known that it will stop distributing XP to U.S. manufacturers and retailers on June 30, while custom system builders can put XP on hardware until Jan. 31, 2009. But what is probably less known -- and not because Microsoft has been hiding the fact -- is that Windows XP Starter Edition, a scaled-down version of the OS for emerging markets, will be available until June 30, 2010.

Moreover, business and end-users who have purchased Windows Vista Business or Ultimate licenses either at retail or through enterprise agreements with Microsoft have indefinite "downgrade" rights to XP as part of their license agreements, according to a Microsoft spokeswoman.

As it's now become clear from internal e-mails made public in a class-action suit based in Seattle concerning Microsoft's Windows Capable program, Microsoft anticipated that customers would experience device-driver and application incompatibilities with Vista when it was first released. Giving buyers of high-end versions of Vista the option to downgrade to XP made sense in case users encountered problems. The availability of Windows Vista Service Pack 1 now is remedying many, if not all, of these issues, users report.

With Microsoft pushing Vista, it's unlikely the company wants XP to be the OS of choice for anyone, even if XP will be in circulation for at least the next few years because of various exceptions.

The company is promoting Vista Starter Edition for inexpensive PCs currently being sold in emerging markets, although that OS may not be a good option for the ultra-low-cost PCs that will run Intel’s Atom chips because of hardware and memory requirements.

Like the other versions of Vista, Starter Edition still requires 15GB of free space; most low-cost machines, however, have only 2-8GB of storage and 256-512MB of RAM. Hackers have managed to get Vista running on the low-cost Asustek Eee PC with 4GB of RAM, but doing so required a complicated custom installation and an additional 1GB USB stick and an 8GB SD card to make it work.

Another thing that's unclear is if Microsoft will let OEMs sell Vista Starter Edition, or an even more scaled-down version of it, in more mainstream PC markets such as North America. So far, ultra low-cost PCs that run Linux like the Eee PC -- which are gaining some, but not much, traction with consumers -- don't seem to be threatening Windows enough in the U.S., at least to require Microsoft to cut Vista's price or footprint even more.

Sumner Lemon in Singapore contributed to this article.