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The well-guarded code will be made available under Microsoft's Shared Source Initiative initially to about 150 licensed systems integrators.
Eligible licensees are Gold Support Services certified partners and system integrators with more than 1,500 seats of Windows with a level A or B Microsoft premier support agreement.
While Microsoft has cast its action as a means to assist system integrators, the announcement's timing suggests other factors may be at work.
Earlier this week, the US federal judge overseeing the ongoing antitrust case against the software maker ordered the company to open source code for recent versions of the Windows operating system to nine states that are plaintiffs in the case. Microsoft's announcement did not mention the litigation.
Microsoft also faces competitive pressure from the open-source Linux operating system. Microsoft's shared-source concept permits integrators and developers to look at Windows code, but not to modify it or resell it. The open-source license covering Linux permits developers to modify the code at will, as long as the modifications are published openly.
"They're trying to make nice from an antitrust perspective, but they're also trying to brunt the force of Linux," said John McCarthy, director of research for Forrester Research.
However, the move is a boon to systems integrators trying to squash bugs they discover while setting up a new system for a customer, said one integrator.
"If we're locked out of the source code, it limits what we can do for our customers," said Scott Weber, a consultant for Weber Systems. "Typically, what we would do is wait for the next service pack or release for our customer," he added.
Steve Ballmer, Microsoft's chief executive officer, has compared open-source software to cancer. The company is concerned that if it uses open-source code in its operating system, it would have to post all of the code on the Internet.
The company also came under fire last year for attacking the disclosure practices of some security companies.
The Shared Source Initiative arose as a counter-measure to Microsoft's image as a secretive company, but only its top partners will have access to Windows code, and they cannot make changes to it.
"They're not going to give the source code to every Tom, Dick and Harry consultant out there," said Weber. "It would cause more problems than it solves."