“Customers have come to us and said they really like the XO laptop and they would like to see Windows on it,” said James Utzschneider, manager of Microsoft’s developing markets unit.

The first of the project’s child-friendly XO laptops running Windows XP will be tested next month in limited trials in four or five countries. Mr. Utzschneider declined to identify the countries, but he said XO laptops running Windows would be generally available by September.The pact with Microsoft is not an exclusive agreement. The Linux version will still be available, and the group will encourage outside software developers to create a version of the project’s educational software, called Sugar, that will run on Windows.

Windows will add a bit to the price of the machines, about $3, the licensing fee Microsoft charges to some developing nations under a program called Unlimited Potential. For those nations that want models that can run both Windows and Linux, the extra hardware required will add another $7 or so to the cost of the machines, Mr. Negroponte said.

The laptops now cost about $200 each, and the project’s goal is to eventually bring the price down to about $100.

O.L.P.C. led the way in designing inexpensive laptops for children in poorer nations, but others have followed, notably Intel with its Classmate PC, which runs Windows and is $400 or less.

The project’s agreement with Microsoft involves no payment by the software giant, and Microsoft will not join One Laptop Per Child’s board. That contrasts with the approach of Intel, which joined the project last July, took a board seat and pledged an $18 million contribution  only to quit in January amid squabbling over Intel’s aggressive sales tactics with the Classmate PC.

Of the Microsoft arrangement, Mr. Negroponte said: “We’ve stayed very pure.”

But the alliance with Microsoft has created some turmoil within the project. Walter Bender, the president who oversaw software development, resigned last month. His departure, Mr. Negroponte said, was “a huge loss to O.L.P.C.”