WASHINGTON -- Microsoft founder Bill Gates took the stand Monday for the first time in his company's epic antitrust case, testifying the penalties being sought by nine states would undermine the Windows software used by millions of Americans.

With his wife, Melinda, in the audience, Gates opened his long-awaited courtroom appearance with a computer-generated slideshow. He demonstrated how the Windows operating system would stop functioning if components such as the Microsoft Explorer Web browser are removed as the states have proposed.

"This shows that if you remove this block of code, other functions are degraded in the most extreme way. They no longer work," Gates said, referring to the removal of the Explorer software.

Dressed in as dark blue suit, Gates took pains to explain the most common technical terms.

In a surprise, the states prosecuting Microsoft chose Steven Kuney, an antitrust expert, rather than their chief lawyer, Brendan Sullivan, to cross-examine Gates.

Windows clones

Gates showed in another demonstration Microsoft's fear that the states' requirements that the company translate its Office business software and give away Internet Explorer would let companies create Windows clones.

To show the companies that could create clones, Gates named five companies that have helped the states' suit: AOL Time Warner, Sun, Gateway, Novell and Oracle.

Kuney questioned Gates' reference to Gateway, a company that builds computers, as a viable operating system maker.

"They're one of the people who would change Windows ... if they had the ability to do so," Gates said.

In written testimony submitted after he was sworn in, Gates argued the penalties the states have proposed would give Microsoft's competitors an unfair advantage.

The penalties include requiring Microsoft to divulge blueprints and technical information about how some of company's products work.

Such penalties would cause a "a massive transfer of Microsoft's intellectual property rights" to competitors, Gates said.

Some settlements

The Justice Department and nine other states have already settled the case and their deal with Microsoft is awaiting court approval. The nine states remaining in the case want tougher penalties than those in the settlement.

Gates said the additional penalties would devalue the Windows operating system, reduce its ability to handle thousands of pieces of computer hardware and software in the marketplace and frustrate Microsoft's incentive to innovate.

The states' proposals "would undermine all three elements of Microsoft's success, causing great damage to Microsoft, other companies that build upon Microsoft's products, and the businesses and consumers that use PC software," Gates testified.

The states say they want Microsoft to disclose some of its blueprints to ensure that software developers can create products that work as well with Windows as Microsoft's own software, reducing the company's advantage over competitors.

"As I understand it, providing Microsoft's technology to its competitors so they can build 'functional equivalents' of our products now, and match all our future innovations for 10 years, is in fact one of the central objectives," Gates said of the states' plan.

The states want U.S. District Judge Colleen Kollar-Kotelly to force Microsoft to create a modular version of Windows that could incorporate competitors' features.

Gates also echoed arguments by Microsoft chief executive Steve Ballmer that a modular Windows requirement is impossible to engineer and would force the company to pull Windows off the market.

States that rejected the government's settlement with Microsoft and are continuing to pursue the antitrust case are Iowa, Utah, Massachusetts, Connecticut, California, Kansas, Florida, Minnesota and West Virginia, along with the District of Columbia.