Microsoft and the European Union have begun a two-day hearing that gives the US giant its last chance to defend itself against possible daily fines.
Microsoft insists it has sufficiently opened up its systems
The software group faces daily fines of up to 2m euros ($2.4m; £1.4m) if it is found to have dragged its feet over a 2004 EU anti-competition ruling.
Microsoft insists it has met the EU's demand that it makes its software work better with that of rivals.
The EU instead currently accuses the firm of delaying tactics.
Under the European Commission's 2004 ruling, Microsoft was told to provide rival firms with more information about its software, so that they can write programs that run more smoothly on Microsoft's ubiquitous Windows operating system.
Neelie Kroes' team is not convinced Microsoft has done enough
"We have complied beyond the requirements of the Commission's decision," Microsoft's top lawyer, Brad Smith, said before the start of the hearing.
"Microsoft is willing to do more... Daily fines are not the solution."
As part of its defence, Microsoft says it has submitted 12,000 pages of documentation to its rivals.
However, European competition officials are so far unconvinced by the firm's arguments.
"Our independent trustee, who is advising the Commission and who was suggested to us by Microsoft, Professor Neil Barrett, has told us the document is, to quote, 'totally useless'," said Jonathan Todd, spokesman for EU Competition Commissioner Neelie Kroes.
He added that following the two-day hearing, the decision on whether or not to invoke the daily fines was likely to take several weeks.
The EU's March 2004 judgement against Microsoft also called for the firm to debundle its Windows Media Player and hit it with a record fine of 497m euros.
Microsoft is due to appeal against the entire judgement in April at Europe's second-highest court, the Court of First Instance.
The company also faces possible anti-competition problems regarding its forthcoming new Vista operating system, which is due to be released early next year.
Earlier this week the European Commission said it had some issues regarding the system which it wanted Microsoft to clarify.