ASHINGTON, May 7 — The federal judge hearing the Microsoft antitrust case said today that she would overlook procedural errors by the states pursuing sanctions against the company and allow them to present new evidence about a restriction they have proposed.

Over Microsoft's objections, Judge Colleen Kollar-Kotelly of the Federal District Court agreed to allow the states to present an expert witness, James Bach, in an attempt to show that the company's Windows operating system can be broken into different components.

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"I think the information should be presented to the court, that I should have it," Judge Kollar-Kotelly said.

The decision was the first indication that the judge, who rarely asks questions and has little background in technology, has been paying close attention to the conflicting testimony over the states' demand for a "modular" version of Windows. This version would allow computer makers to substitute rival software for programs like Microsoft's Web browser.

The states say Microsoft should be made to produce such a program to restore competition after an appeals court ruled last June that the company violated antitrust laws to defend its monopoly on operating systems.

Several Microsoft witnesses, including the company's chairman, Bill Gates, have argued that Windows cannot be separated into its component parts without significant damage to what remains. But the states have suggested that Microsoft already makes a modular version of the operating system in Windows XP Embedded, currently licensed only for use on special-purpose devices.

A lawyer for the states said that Mr. Bach, a software testing consultant and former Microsoft employee, would show a video to demonstrate that XP Embedded can be used to power personal computers.

Judge Kollar-Kotelly rebuked the states' lawyers for waiting until the eighth week of the hearing to make their request. Since they hired Mr. Bach last February, she added, it was "somewhat disingenuous" for the states to say they thought of calling him as a witness only after they heard the evidence presented by Microsoft.

But Microsoft may still have out-maneuvered the states procedurally. By deciding not to call several company executives who were previously on its list of witnesses, Microsoft appears to have blocked the states from using a dozen documents that might have bolstered their case. The states have requested that the documents be admitted.

Jim Allchin, a Microsoft group vice president in charge of Windows, argued that the company's proposed settlement with the Justice Department would allow it to share more information with software developers while still protecting Windows from security attacks.

But the states want Microsoft to disclose more information than the settlement required.