Microsoft has until the end of May to come to terms with EU authorities over a long-running antitrust dispute, the European Commission has said.
Microsoft is dragging its feet, the European Commission insists
The deadline refers to a 2004 judgement, where the Commission found the software firm guilty of abusing its market dominance.
Microsoft is supposed to provide details of how it could bring its practices back in line with EU rules.
But Brussels has repeatedly expressed dissatisfaction at the slow pace.
"We are waiting for the Microsoft people to do their homework," said European Competition Commissioner Neelie Kroes
A Microsoft spokesman in Brussels said the software giant would "continue to work out with the commission towards an agreement on full compliance with the decision".
In addition to a 497m-euro ($640m; £343m) fine in March 2004, Brussels ordered Microsoft to open up its core software systems to rivals.
This was to enable other software manufacturers to make programs that worked more seamlessly with Microsoft's Windows operating system.
Microsoft was also ordered to provide a version of Windows without its own Windows Media Player, to give rival software makers a chance to compete, and computer makers the option to supply PCs with the media software of their own chosing.
Under EU rules Europe could fine Microsoft up to 5% of its daily global turnover for each day that a decision is not applied to its liking.
Ms Kroes said it was "too early" to say whether the case against Microsoft could be reopened.