Microsoft Corporation is trying to assess damage from a leak of computer code for its Windows operating system over the Internet. Analysts say it is too early to know how much harm the leak could cause.

Microsoft says parts of the source code for its Windows 2000 and Windows NT 4.0 operating systems were illegally made available on the Internet. A company statement issued Thursday says Microsoft is working with federal investigators to determine the source of the leak, which analysts say could potentially let hackers exploit weaknesses in the operating systems. The release of millions of lines of computer code could also help the company's rivals by exposing the inner workings of Microsoft systems.

Microsoft products are used in 90 percent of the world's personal computers.

Some of the software giant's critics say Microsoft products have an inherent weakness because their code is secret. Douglas Thomas, a computer security expert at the University of Southern California, says open-source systems like Unix and Linux rely entirely on software design for their security.

Mr. Thomas says it is too early to know what damage was done to Microsoft Corporation, or to users of the affected products. "The best-case scenario is if it's a well designed system, then the risks should be insignificant or negligible," he said. "The worst-case scenario is, if it's a poorly designed system, then the potential for disaster is enormous because you've just told the world, for example, that the front door key is hidden under the mat."

Analysts say the potential damage will depend on what parts of the code were released and on the strength of security systems built into the software.

Microsoft has shared some of its closely guarded code with U.S. government agencies, with other companies and with some of its clients. The company statement says it does not appear that the leak was a breach of Microsoft's corporate network or internal security, and it says there is no known impact on customers.