In a second day of high courtroom drama, the government exposed new, embarrassing problems Wednesday with a digital video demonstration prepared by Microsoft to buttress one of its bedrock legal defenses.

Over a four-minute segment, Justice Department lawyer David Boies pointed out software icons that appeared — then disappeared and reappeared — on the screen of a computer purported to have been tested by Microsoft.

U.S. District Judge Thomas Penfield Jackson grabbed his brow and shook his head in exasperation to admonish James Allchin, a Microsoft executive on the witness stand, about the video.

Allchin, a top scientist and a senior vice president, was forced to explain that Microsoft had tested several computers with different software packages, but edited together video segments so it appeared a single computer was being tested during the demonstration.

"This wasn't in our lab trying to be incredibly precise," Allchin said.

"How can I rely on it if you can't tell me whether it's the same machine or whether any changes have been made to it?" the judge demanded. "It's very troubling, Mr. Allchin."

Later, in a private meeting with lawyers, the judge said he did "not believe that (Allchin) deliberately falsified this, but it does cast serious doubt on reliability of that exhibit altogether," according to a court transcript.

The judge added that Boies "has done a very professional job of discrediting those tapes."

Struggling to recover its credibility following the courtroom fiasco, Microsoft promised to re-create the demonstration overnight Wednesday in downtown Washington with government experts watching. They planned to play the new video for the judge today.

Microsoft had believed its demonstration would be an effective rebuttal of the government's claims that its Internet browser software could be easily removed from Windows 98. The video shows its browser running dramatically slower after the government tried to disable most Internet functions in Windows.

The issue is central because the government alleges that Microsoft illegally "tied" its browser to newer versions of Windows, forcing consumers who use its popular computer operating system also to use its Internet software.

Pitting its technical expertise against Microsoft — with its legions of brilliant programmers — the government hired as consultants Christian Hicks and Peter Creath, both in their early 20s, and their former computer professor, Edward Felten of Princeton University.

The three studied Microsoft's demonstration video ever since the company handed over a preview copy to government lawyers just over a week ago.

They noticed a subtle flaw in the video that Boies used to confront Allchin about on Tuesday with Perry Mason-style fanfare, then they discovered new problems when they reviewed the demonstration again Wednesday.

Boies gave credit to his young computer experts. While he's nationally known for his legal skills, Boies has proved less than adept tackling technical issues.

He once mispronounced "log in" as "lojun," and frequently calls the world's largest Internet provider "American Online" instead of "America Online."

Allchin had testified he was assured by his "total trust in my team" that Microsoft's tests had been performed using clean computers containing only Windows software with the government's modifications.

But Boies pointed out telltale remnants of other programs, such as parts of Microsoft's Office business software. In one segment, nine icons were visible in the background on the computer's screen; in another, one was missing. Later, it reappeared.

"Clearly the impression given is, this is the same machine," Boies charged.

"We make very good software. We didn't make a very good tape," said the company's lead in-house lawyer, William Neukom. "It's frankly our fault. We've got to do a better job."