The One Laptop Per Child project is about to find out whether Microsoft Corp., a rival the nonprofit group once derided, is the solution to its problems in spreading inexpensive portable computers to schoolchildren.

Microsoft and the laptop organization announced Thursday that the nonprofit's green-and-white "XO" computers now can run Windows in addition to their homegrown interface, which is built on the open Linux operating system. That had been anticipated for months, but it amounts to a major shift.

Nicholas Negroponte, the founder of the laptop project _ which aims to produce $100 computers but now sells them at $188 _ acknowledged that having Windows as an option could reassure education ministers who have hesitated to buy XOs with its new interface, called Sugar. Negroponte had hoped to sell several million laptops by now; instead he has gotten about 600,000 orders.

Beginning in limited runs next month, XO buyers will have the option of computers loaded with or without Windows. Versions with Windows will cost $18 to $20 more; $3 of that is for Windows, and the rest covers hardware adjustments, like an additional memory-card slot, needed to make Windows run.

Soon Negroponte hopes to sell just one kind of machine with a "dual-boot" mode, meaning users would have Windows and Linux and choose which to run each time. Because that will take advantage of a broader hardware redesign, the dual-boot XOs will cost about $10 more than today's versions, Negroponte said.

Despite the higher price _ and Windows' inability to take advantage of some key features of the XO _ Negroponte said his project would benefit from Microsoft's strengths in selling and deploying technology.

"I think our goals are dramatically enhanced with Microsoft's decision and this partnership because we will reach many more children," he said. "There are now many more countries prepared to look at the XO and collaborative learning and some of the things we stand for."

The partnership culminates an odd dance.

Not long after Negroponte first dreamed up the idea of seeding the developing world with $100 laptops for education, he talked with Microsoft about using a version of Windows on the machines. That seemed to vanish before long, as Microsoft's Bill Gates and a close partner, Intel Corp. Chairman Craig Barrett, publicly dismissed the XOs' scaled-back processing power and small screen.

At first Negroponte wore the criticism as a badge of honor, saying it showed that his little group would upend the laptop market. "When you have both Intel and Microsoft on your case, you know you're doing something right," Negroponte said to cheers at a Linux convention in 2006.

Negroponte had other reasons for pursuing a path separate from Windows. For one, Linux is free. That's key when you're trying to make a computer for $100. Plus, Linux was seen as easier to configure for the XOs' specific innovations, such as its ultra-low power consumption.

Negroponte and his crew also talked about how the open nature of Linux better suited the project's vision for "constructivist" learning, with children teaching each other and themselves by tinkering with the computer. Negroponte has said he finds it sad when children learn to use computers mainly as tools for office automation.

"The hundred-dollar laptop is an education project," he often said. "It's not a laptop project."

However, it's enough of a laptop project that Negroponte is eager to speed XO sales and donations beyond their initial deployments, which include Haiti, Afghanistan, Rwanda, Mexico, Peru, Uruguay, Mongolia and Birmingham, Ala.

Negroponte's first big change was to make peace with Intel last year, in hopes of boosting XOs' technical development and blunting competition from Intel's Classmate PCs. But the relationship ended after only a few months.

The Microsoft relationship looks sturdier. Microsoft engineers spent the past year customizing a version of Windows that can run on XOs. Even so, XOs running Windows for now can't use some of the machines' security features and their built-in "mesh" wireless networking.

Negroponte indicated last month that eventually, Windows could be the sole operating system, with Sugar serving as educational software running on top of it. But he said Thursday he does not envision that happening.

Still, a key question will be whether having Windows on the laptops means children make less use of Sugar, one of the project's core innovations. Recently a splinter group formed to keep up development of Sugar, and Negroponte is enduring complaints that education is no longer his top priority.

"OLPC changed its mission outright, and in the most ill-conceived way imaginable," Ivan Krstic, a former security developer for the laptop group, recently wrote in an e-mail.