Microsoft has opened its attack against European Union antitrust findings that it withheld information from rivals, as a court hearing pitting the software giant against the EU entered a third day.

Microsoft lawyer Ian Forrester told a 13-judge panel of the Court of First Instance, the second highest EU court, that the company’s actions were entirely legal. He accused the EU's executive commission of interfering with market forces.

"We... submit that the decision is an attempt to reconfigure [how the] market works by handicapping the leading player in perpetuity," Forrester said.

The Commission found in 2004 that Microsoft violated EU antitrust laws, fined it €497m (about £345m) and ordered it to change its business practices.

Attention turned to a commission finding that Microsoft failed to provide needed information to rival makers of work group servers, used by small groups within companies to access files, sign-on and print.

The finding says Microsoft must make available enough information so that competitor products can hook up as smoothly with desktop Microsoft computers as the company's own server software.

Forrester said that the law alone was enough to sink the Commission's findings, adding that interoperability was not the problem it was made out to be by the Commission.

"Interoperability already exists," Forrester said.

He said the commission required Microsoft to license its confidential information, in contrast to earlier cases where compulsory licensing involved well-known information, such as the shape of a Volvo car door or a republication of television listings.

A Microsoft executive, John Shewchuck, gave the judges an elaborate slide presentation that demonstrated Microsoft's view of how server compatibility works with the Windows operating system, which runs on 95 percent of the world's PCs.

"Microsoft spends an enormous amount of effort attempting to achieve interoperability," said Shewchuck. "Interoperability is commonplace."

His slides showed how systems can be configured to work together in most circumstances.

A group of companies critical of Microsoft said in a statement that the case originated because Microsoft had failed to provide needed interconnection information on reasonable and non-discriminatory terms.

"Novell, not Microsoft, created this technology," said the group, but "work group servers remained dependent on the willingness of Microsoft to supply" needed interconnection information.

On Monday and Tuesday, Microsoft attacked Commission findings that it illegally bundled streaming audiovisual software in its Windows operating system.

The court is expected to take months to come to a decision.

Click here for the best Microsoft voucher codes.