It's a rare day in the Microsoft trial when the defense lets a government witness go quickly. For most of the 10 witnesses, cross-examination has dragged on for torturous days. But on Dec. 14, Microsoft attorneys saw that they weren't getting anywhere in their attempts to pierce the testimony of Princeton University professor Edward W. Felten -- and that their line of questioning was trying the judge's patience. By 3 p.m., Microsoft's lawyers called it a day, and Felten was free to go.

Felten, a boyish-looking 35-year-old, took on Microsoft's central argument that Internet browsing was such an integral function of the Windows 98 operating system that it was impossible to remove without breaking the system. The government alleges that Internet Explorer and Windows 98 are separate products and that Microsoft combined them mainly as a way to dominate the browser market, since the Internet software would be included without charge in every new copy of Windows. Microsoft argues that this so-called bundling is simply the natural evolution of Windows: Browsing is just one more function of the operating system.

The most dramatic testimony was a half-hour videotaped demonstration that the government showed in court. It involved a computer screen and a "prototype" computer program developed by Felten and two graduate students. As Felten looked on from the witness stand, he argued in the video that the program was able to remove "all the methods to launch Web browsing" from Windows 98 without affecting the stability of Windows. The video showed the various ways that a user can get into the Internet with Windows 98 and then showed that users could no longer use these methods once Felten's program was installed. After he removed Internet Explorer, he installed a copy of Netscape Communications Corp.'s Navigator browser. He said Navigator installed this way "substantially duplicates" for the user the experience of Windows 98 with Internet Explorer.

HIDE AND SEEK. Felten said during cross-examination that computer makers -- if given a choice -- could develop a package with Windows 98 and Navigator that is the same as the Windows 98 with the Explorer. He also noted that he knew of "no reason why Microsoft was technically compelled to design" Windows 98 the way it did.

During his questioning, Microsoft attorney David Heiner tried to make the point that Felten only hid the code that enabled browser technologies to work and did not remove the browser code altogether. He tried to pin Felten down on the exact code that made the browser a separate product. He noted that the same four files that contained browsing functionality also ran other parts of the operating system, and that if he really removed the code in these files, the system would fall apart. Felten argued repeatedly that just because Microsoft decided to "package" the code in this way doesn't mean that Microsoft can't remove certain pieces that allow for Web browsing without hurting the rest of the operating system.

As the back-and-forth continued, U.S. District Judge Thomas Penfield Jackson became so exasperated that he stopped Heiner in his tracks, accusing him of "playing word games." Said the judge: "He's told you at least a dozen times that there is code in each of the files, some of which pertains to browser functionality and also other code not related to the browser. To pursue this in the hope that he will make a slip of the lip, I don't think is an appropriate cross-examination."

LEGALLY INTEGRATED? Whether Felten's testimony makes a difference in the trial -- or in a subsequent appeal -- is an open question. Last June, ruling in a related Microsoft case, the circuit court indicated that it believed the browser and the operating system were legally integrated in Windows 95. It also ruled that companies should have wide latitude in deciding how to design their products and that courts should be "wary of second-guessing the claimed benefits of a particular design decision." But in the government's favor was a statement by the appeals court that the "combination offered by the manufacturer must be different from what the purchaser could create from the separate products on his own." Felten's testimony seemed to address that concern by saying that a PC maker could achieve the same results both ways.

If Justice wins, however, Felten's demonstration could become a factor in the remedy phase -- because the government has indicated that it wants Microsoft to remove Internet Explorer from Windows 98 or to ship Navigator with Windows, too. Microsoft has argued that such a move was technically impossible. Now, Justice has shown, via Felten, that there is a way to do it.

Meanwhile, stay tuned for more TV tidbits: On Dec. 15 and Dec. 16, the government plans to show more excerpts from Microsoft Chairman Bill Gates's videotaped deposition and also excerpts from depositions of executives from PC makers Gateway and Packard Bell.

By Susan Garland in Washington