The decision means that the software giant moves one step closer to a fine of up to €2m a day.
Microsoft had requested access to correspondence involving technical experts for its defence, which was also turned down by the Commission.
"We have confirmed the deadline of 15 February on the basis that nearly eight weeks should be sufficient for Microsoft to reply to a comparatively short statement of objections," said Commission spokesman Jonathan Todd.
Todd explained that Microsoft had been given access to some third-party correspondence, because the people involved had waived their right to confidentiality.
In a quick reply, Microsoft focused on the denial of access to correspondence between the Commission, a monitoring trustee and other technical experts.
"The Commission's position on this point is contrary to both the letter and spirit of the law," wrote Ian Forrester, a Microsoft lawyer, to Hearing Officer Karen Williams.
Earlier, Microsoft Europe's associate general counsel, Horacio Gutierrez, said: "The Commission cannot unilaterally take away a fundamental right of defence."
But Todd insisted otherwise. "In contrast to Microsoft's claims, the hearing officer considers that Microsoft's rights of defence have been properly safeguarded," he said.
In November last year, the Commission ruled that Microsoft had failed to comply with remedies stipulated in a landmark 2004 antitrust decision. The company was fined €497m, which it has paid.
The Commission ordered Microsoft to provide workable instructions to enable competitors to develop server software that worked with Microsoft's Windows operating system as efficiently as Microsoft's programs did.
However, the independent trustee considered the documentation Microsoft supplied as "fundamentally flawed in its conception".
The Commission gave Microsoft until 15 December to comply with the remedy or face a new daily fine.
As that deadline has passed, the Commission has now opened new proceedings to impose a daily fine, based in part on the findings of the trustee, who was chosen by the Commission from several candidates nominated by Microsoft.
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