The European Commission has threatened to fine Microsoft up to 2m euros ($2.4m) a day until it gives rivals more access to its operating systems.
Brussels said the software giant had failed to supply adequate information about its server programmes.
Microsoft has five weeks to provide improved documentation before the daily penalties are imposed.
The EU said it was “left with no alternative” after giving Microsoft every chance to comply with its ruling.
In a landmark ruling in March 2004, the world’s biggest software firm was found guilty of abusing its position, hit with a record fine and told to open up its systems.