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

Microsoft and the laptop organization announced May 15 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, such as 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 XO's 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 XO's specific innovations, such as its ultra-low power consumption.