In 10 weeks, Microsoft Corp. will begin to retire Windows XP by shifting the seven-year-old OS into a more limited support plan.

Windows XP, Microsoft's most successful operating syste m ever, will leave what the company calls "mainstream support" on April 14, and enter "extended support." Typically, Microsoft keeps a product in the former for five years, then moves it into the latter for another five, for a total of 10 years. However, the long span between the releases of XP and its successor, Windows Vista, forced the company to push out the support deadline to 13 years altogether.

Also, two years ago Microsoft bumped support for Windows XP Home and XP Media Center to the 2009 and 2014 dates, matching the dates that had previously been set for Windows XP Professional, the designated business edition of the operating system.

By Microsoft policy, mainstream support delivers free fixes -- for security patches and other bug fixes -- to everyone. During extended support, all users receive all security updates, but non-security hot fixes are provided only to companies that have signed support contracts with Microsoft.

Several Microsoft spokespeople confirmed that Tuesday. "Customers will have access to extended support for paid support, security support updates at no additional cost and paid hotfix support," a company spokeswoman said in an e-mail. Firms must purchase an extended support contract within 90 days of XP's mainstream support retirement in April.

"All security updates are provided through both mainstream and extended support," added Frank Fellows, another Microsoft spokesman.

Although it's not unusual for a version of Windows to be still in widespread use when it moves into extended support, XP is a unique case, said Michael Cherry , an analyst with Directions on Microsoft. "This is the first time I can remember that we have a situation where people will be continuing to buy devices with an operating system no longer in mainstream [support]," said Cherry.

The devices he was referring to are netbooks, the loose category of low-priced, small-sized laptops that accounted for a significant portion of PC sales in the last few months of 2008. About 80% of all netbooks sold in the last quarter shipped with a copy of Windows, Microsoft claimed last month. The bulk of those netbooks shipped with Windows XP; with its bigger footprint and heartier system requirements, Vista can't be squeezed into most low-end laptops.

Last year, Microsoft extended XP's sales lifespan specifically to account for netbooks, pushing the drop-dead date out to mid-2010.

"If you're buying a netbook with XP, you have to accept that XP is not in mainstream support," Cherry added. Not that that should matter much. "XP is well known by this point," Cherry argued. "A significant number of its problems have been identified and resolved, so the chances aren't great that there would be some new major issue."

Microsoft has, in fact, issued a total of three service packs for the aged operating system, the most recent, Windows XP SP3 , in May 2008.

"But you also have to look at the reality of the marketplace," Cherry cautioned. "Once XP is not in mainstream support, Microsoft is not going to make any functional enhancements to XP. If there's a functionality bug with no security issue, it probably won't get fixed."

For its part, Microsoft downplayed the impact on users who purchase new systems powered by XP after the operating system leaves extended support. "For any copy of Windows XP that you buy pre-installed, the OEM will provide the support," the company's spokeswoman said. "This support is not tied to the Microsoft Support Lifecycle policy, but rather to the OEM's support policy. So, if a consumer purchases a netbook today with Windows XP Home pre-installed, their primary support would be through the OEM."

This story, "Windows XP's Days are Really Numbered Now" was originally published by Computerworld .