The judge in Microsoft's antitrust trial has likened the firm's control of its Windows software to John D Rockefeller's Standard Oil monopoly of the 19th century, thereby casting doubt on a company defence.

"Mr Rockefeller had fee simple (absolute) control over his oil," District Judge Thomas Penfield Jackson told Microsoft lawyer, Mr John Warden.

"I don't really see a distinction," the judge said of the comparison with the Windows operating system used on most personal computers.

Judge Jackson was speaking at a hearing to help determine whether Microsoft's action violates the Sherman Antitrust Act of 1890 - a law prompted in large part by the abuses of the Standard Oil trust.

Last November, Judge Jackson found that Microsoft held monopoly power and had used it to harm consumers, rivals and other companies - but he stopped short of any legal conclusions.

Mr Warden had argued that Microsoft was justified in taking actions the government says were illegal, because it enjoys copyright protection for Windows. He argued copyright protection gave Microsoft the right to limit computer makers' use of its product.

"What evidence did you give me about what is protected by copyright?" Judge Jackson asked.

Warden said that the company had filed its copyright registration and that the burden had shifted to the government.

"I don't really understand your copyright defence," Judge Jackson said.

Earlier, government lawyer, Mr David Boies, said that Microsoft had never supported its copyright allegations with any evidence.

Mr Boies compared the judge's findings of fact to a string of Supreme Court decisions which he said proved the world's largest software company broke antitrust laws.

Mr Boies also cited Supreme Court cases which concluded that the Sherman Act prohibits conduct that has no legitimate purpose except to exclude rivals.