Microsoft lawyers are expected to open a new front in their battle against a European Commission competition ruling. On the third day of a key anti-trust hearing, the US software giant will challenge an EU finding that it failed to open up its software to rivals.
Microsoft is in Luxembourg to appeal against a landmark anti-trust ruling handed down by the Commission in 2004.
Brussels fined Microsoft 497m euros ($613m; £344m) and ordered it to change its business practices.
Microsoft's challenge at the Court of First Instance could have widespread ramifications for future competition rulings by the Commission if the court's 13-member panel rule in its favour.
Lawyers representing Microsoft are planning to appeal against a finding by EU regulators that the US firm failed to provide rivals with enough information to develop software that could run as smoothly as its own on Microsoft's Windows operating system.
Earlier in the week, the firm challenged a ruling that it was damaging competition by bundling its own Media Player software with its Windows package - which is used by about 95% of the world's personal computers.
Microsoft argued that the ruling would stifle innovation and accused the Commission of seeking "a dramatic change" in its business model.
But lawyers for the Commission said that unless Microsoft gave its rivals greater access to its programming code and stopped selling its Media Player as part of Windows, rivals may be forced out of business.
Per Hellstrom, a lawyer for the Commission, said that Microsoft's size and near monopoly position meant that the case had "vital importance for innovation".
"Microsoft cannot be allowed to decide who can innovate and who cannot," he said.
Microsoft earlier claimed that the Commission's decision to force it to offer a version of Windows without any media player system - known as XPN - had failed.
On Monday, lawyers for the firm told the court that XPN sales represented 0.005% of overall XP sales in Europe.
Microsoft's appeal is being heard over five days by the Court of First Instance, although a decision in the case is not expected for months, or possibly a year.
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