Microsoft Corp. on Thursday confirmed that the source code for two versions of its Windows operating system has been leaked, a security breach that could give hackers important intelligence about how to exploit flaws in software run by most of the world's computers.

"Today we became aware that incomplete portions of Windows 2000 and NT 4.0 source code was illegally made available on the Internet," said Microsoft spokesman Tom Pilla. "It's illegal for third parties to post Microsoft source code and we take that activity very seriously."

Pilla said the company does not know how much of the operating system code was compromised, but he said Microsoft believes it was not a complete version of either operating system.

There was no indication that the code was stolen through a breach of the Redmond, Wash.-based company's internal network, Pilla said. He said the FBI is investigating the matter.

Computer security experts said the release of Windows source code could pose a significant threat to Internet security, depending on what portion of the code was leaked.

A leak of any portion of the Windows code "could dramatically increase the probability that new zero-day vulnerabilities will be found," said Alan Paller, director of research the SANS Institute, a security training group based in Bethesda, Md.

"Zero day" exploits are highly effective attacks that occur when hackers discover a way to exploit a security vulnerability before or at the same time as a software maker learns of the flaw. Attackers can then use this information to launch a virus or worm that exploits the security hole before a patch can be released to fix the problem.

Thor Larholm, senior security researcher at Newport Beach, Calif.-based PivX Solutions, said the Windows source code file being traded on the Internet appears to be roughly 660 megabytes in size, about the size of one CD-ROM's worth of data. That is far short of the estimated 40 gigabytes of data that makes up the entire 40 million lines of code in the Windows operating system.

Even a partial leak "is a potentially very serious problem for Microsoft," Larholm said. "Just look at the vulnerabilities that are discovered by people who didn't have access to the source code."

Howard Schmidt, former head of security at Microsoft, said he was less concerned about the security implications of the leak than its potential threat to Microsoft's intellectual property.

"From a security standpoint, this is sort of like capturing a 1956 Russian fighter jet," he said. "Everyone has been beating on Windows 2000 and NT for a long time, and any flaws that may be found have likely been fixed long ago. Frankly, I'd be more worried that someone was going to use this as a base for developing software or another operating system based on Microsoft's proprietary code."

Schmidt left Microsoft in December 2001 to work as a cyber-security official in the Bush administration. He is currently chief security officer at online auction giant eBay.

Microsoft closely guards the computer code that comprises the company's operating system. But Microsoft does license portions of its programming code to security researchers and more than 50 universities under its "Shared Source Initiative."

Microsoft last year said it would began sharing complete copies of its source code with governments around the world that want to validate the security of the software before deploying it in national defense and other sensitive areas. Microsoft signed an agreement in 2003 that lets the Australian government inspect the source code of Windows 2000, Windows XP and Windows Server 2003. Other counties, including India, are exploring similar arrangements.

Unlike open-source software like the widely used Linux operating system, the code comprising Microsoft's Windows software is not open for public inspection. Linux users are encouraged to participate in an open, continuous cycle of modifications and upgrades that its proponents say results in systems that are more secure and reliable than those powered by proprietary code like Windows.