The European Commission levied a "deplorable" penalty because it was "coloured by what people would think, " Ian Forrester, representing Microsoft, told the European Court of First Instance in Luxembourg.

Microsoft, whose Windows operating system runs on about 95% of the world's personal computers, is asking the court to cancel the fine and overturn the EU's order. Forrester said the ruling draws "a new line" in EU law, one the Washington-based company couldn't have known it was crossing.

The commission may consider that lawyers of big companies are "better able to predict the unpredictable, " Forrester told the court.

This is not the case, he said.

Commission lawyer Fernando Castillo de la Torre dismissed Microsoft's claims however. "Generally large firms are more aware of antitrust behaviour, " Castillo de la Torre said. "If the commission must impose deterrent fines, it must be commensurate with the size of the company."

Friday's hearing was the fifth and final day. The two sides have battled all week on aspects of the ruling, including an order to strip Microsoft's media player from a version of Windows and share data with competitors.

The 13-judge panel last week considered the appointment of an independent trustee and whether to reduce or cancel the fine.

"Any reduction of the fine will send a message that a big company can irreversibly distort a market by just paying a fee, " Castillo de la Torre told the court. Both sides also addressed the appointment in October 2005 of computer scientist Neil Barrett to provide "impartial advice" on Microsoft's compliance with the March 2004 decision.

Judge John Cooke, who will draft the panel's decision, asked the commission whether it was not concerned that the monitoring had been outsourced to a trustee whose cost Microsoft must bear, potentially indefinitely.

Commission lawyer Anthony Whelan said the trustee, the first ever appointed in an EU antitrust case, is necessary to ensure Microsoft's abusive behaviour ends.

Microsoft argues the appointment is unlawful as the commission doesn't have the power to delegate enforcement powers to a private person. "Such a requirement is beyond the commission's power and therefore illegal, " Jean-Francois Bellis, representing Microsoft, told the court.

The commission also lacks the authority to impose on Microsoft the costs of the trustee, Bellis said, arguing that "the commission fails to identify any legal basis" for this action.

The court is led by Danish Judge Bo Vesterdorf, who in December 2004 rejected Microsoft's request to suspend the EU decision.

A final ruling, which is expected next year, can be appealed at the European Court of Justice, the EU's highest court.