Microsoft has appealed against a 899m euros ($1.4bn; Ј680.9m) fine given for defying sanctions imposed on it for anti-competitive behaviour.
The penalty - the largest ever from the European Commission - came after it failed to comply with a 2004 ruling that it abused its market position.
The ruling said that Microsoft was guilty of not providing key code to rival software makers.
Microsoft said it was appealing to seek "clarity from the court".
The challenge has been lodged with the EU Court of First Instance.
Freezing out rivals
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The fines came on top of earlier fines of 280m euros imposed in July 2006, and of 497m euros in March 2004.
An investigation concluded in 2004 that Microsoft was guilty of freezing out rivals in products such as media players, while unfairly linking its Explorer internet browser to its Windows operating system at the expense of rival servers.
The European Court of First Instance upheld this ruling last year, which ordered Microsoft to pay 497m euros for abusing its dominant market position.
Earlier this year, Microsoft announced that it would open up the technology of some of its leading software, including Windows, to make it easier to operate with rivals' products.
(BBC)
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