Microsoft begins its appeal today against a European Commission decision to fine it millions of euros for allegedly breaking EU competition law.
The commission found last year that Microsoft had abused a virtual monopoly in its Windows operating system to muscle out rivals.
It imposed a record €497 million ($613 million) fine and ordered Microsoft to change the way it sells software.
Microsoft is now challenging that decision before the European Union's second-highest court, the Court of First Instance. The court is holding the five-day hearing before its rarely used Grand Chamber of 13 judges.
The first two days will be devoted to a review of the commission's decision that Microsoft acted illegally by integrating audiovisual software into the Windows operating system, found on more than 90 per cent of personal computers worldwide.
On Wednesday and Thursday the judges will hear Microsoft's challenge against the commission's finding that Microsoft deliberately withheld information from rival makers of work group servers, so their software would not work as well as Microsoft's own.
Work group servers are used for printing, file-access and sign-on functions. Microsoft's share of the market has climbed steadily since it began competing illegally, the commission said.
But Microsoft says it acted legally, that others can compete fairly and the court should throw out the commission decision.
A decision in the case is not expected for months, possibly a year.