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.

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.

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.

The Justice Department and nine other states have already settled the case and their deal with Microsoft is awaiting court approval.

The nine states, including Kansas, remaining in the case want tougher penalties than those in the settlement.