Microsoft's Windows 95 rollout presented the most challenges in the company's history, leading to several last-minute changes to technical features that would no longer support a rival software maker's word processor, Bill Gates testified Monday in a $US1 billion antitrust lawsuit filed by the creator of WordPerfect.

"We worked super hard," the Microsoft co-founder said. "It was the most challenging, trying project we had ever done."

Microsoft founder BIll Gates arrives at court.

Gates was the first witness to testify Monday in the US as Microsoft lawyers presented their case in the trial that's been ongoing in federal court in Salt Lake City for about a month. He is set to resume testimony Tuesday morning.

Utah-based Novell sued Microsoft in 2004, claiming the Redmond, Washington, company violated US antitrust laws through its arrangements with other software makers when it launched Windows 95. Novell says it was later forced to sell WordPerfect for a $US1.2 billion loss. Novell is now a wholly owned subsidiary of The Attachmate Group, the result of a merger that was completed earlier this year.