The European Commission has threatened to fine the software giant, Microsoft, up to €2 million a day until it gives rivals more access to its systems.
The commission said the company had failed to supply adequate information about its server programs.
Microsoft has five weeks to provide improved documentation before the daily penalties are imposed.
The group has said it would contest the EU's demands by whatever means possible.
The commission ruled in 2004 that Microsoft had abused its global dominance by leveraging its near monopoly in the market for PC operating systems and for media players to defeat rivals.
It imposed a record €497 million fine and forced Microsoft to sell a version of Windows without the Windows Media Player software used to watch films and listen to music, giving rivals a fairer chance to compete.
Microsoft said in a statement that the commission's latest demand 'can open the door to the production of clones of parts of the Windows operating system' and went beyond the scope of the original EU decision.