WASHINGTON (CNN/Money) - Microsoft and RealNetworks debated Thursday whether RealNetworks' media player would function if Microsoft excised its own media player from its Windows operating system.

Will Poole, who oversees Microsoft's digital media and anti-piracy divisions, testified in federal court that RealOne, the latest digital media program from RealNetworks, relies on key features in Windows.

"As a result, RealOne Player would not run on a version of Windows from which all of the multimedia (interfaces) have been removed," Poole testified. That might occur if computer manufacturers were allowed to remove Microsoft's Windows Media Player.

RealNetworks vice president Dan Sheeran disputed Microsoft's assertion. Sheeran said RealOne relies on operating system functions, not portions of Windows Media Player.

Microsoft is "clearly trying to mislead the court," Sheeran said in an interview.

Nine states have asked the court to force Microsoft to release a modular version of its Windows operating system in which some features, like the media player, could be removed and replaced with competitors' products.

The two companies differ, both in the courtroom and out, on what the state proposal requires Microsoft to remove. The states and RealNetworks say that basic sound and CD playback functions would be allowed to remain in Windows, as they existed before Windows Media Player was introduced.

Microsoft says that the proposals are written so broadly that those functions would have to go.

"We'd be happy to teach Microsoft how to remove Windows Media Player from Windows if they need to," Sheeran said.

Poole also disputed RealNetworks' allegations that Microsoft's new Windows XP favors Microsoft's media player even if a user indicates a preference for another product.

There are over 250 million users of RealNetworks' players, Poole said, and most of them use Windows.

"The fact that so many users choose to use RealNetworks' media players...is evidence that RealNetworks' media players work very well on Microsoft's operating systems," Poole testified.

Over 90 percent of personal computers run Windows operating systems.

RealNetworks vice president David Richards testified in March that when consumers try to create music CDs or play digital music, Windows XP tends to use its own media player even if the user selected the RealNetworks product as the default player.

The original judge in the antitrust case, Thomas Penfield Jackson, ordered Microsoft broken into two companies after concluding that it illegally stifled competitors. An appeals court upheld many of the violations but reversed the breakup order and appointed U.S. District Judge Colleen Kollar-Kotelly to determine a new punishment.

Other penalties sought by the states would require Microsoft to license its Office business software for use on competing operating systems and distribute the blueprints of its Internet Explorer browser for no charge.

A computer expert testifying on behalf of Microsoft Corp. in the ongoing antitrust action stumbled several times while on the stand Wednesday.

"I'm not trying to be evasive," Stuart E. Madnick, a computer and software expert and professor at MIT, said at one point during the testimony. "I'm just trying to be precise."

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One of the key issues in the antitrust case revolves around Microsoft's incorporation of the Internet Explorer Web browsing software into its Windows operating system in such a way that IE can't be removed without damaging Windows. The courts found that Microsoft has monopoly power in the market for desktop computer operating systems, and violated antitrust law to protect that power.

Critics, including the dissenting states, have argued that Microsoft deliberately designs its products to interfere with technology made by other companies, forcing people to use Microsoft products. The dissenting states are pressing for complete interoperability. But Madnick argued that being able to exchange and use data on any level -- even if the process is clumsy -- is enough to claim interoperability.

When Madnick returned to the stand Thursday, he said it would be possible to create a modular Windows, although he didn't think it was a good idea.

He said Microsoft could replicate necessary portions of the operating system before taking out a feature like the Web browser. But he guessed that the change could make Windows "100 to 1,000 times more bloated."

"You can do it, but it doesn't make much sense," Madnick said.

--from staff and wire reports