EU hits Microsoft with a deadline
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.
In addition to a 497m euro ($640m; £343m) fine in March 2004, Brussels ordered Microsoft to open up its core software systems to rivals.
Ms Kroes said it was "too early" to say whether the case against Microsoft could be reopened.