Microsoft has not done enough to comply with sanctions imposed for breaking European anti-trust rules, the European Commission (EC) has said.
Microsoft is under pressure to close security holes
The software giant was censured in 2004 for misusing its monopoly position in desktop PCs to extend its reach into other areas.
The firm agreed to a 497m euro ($660m, £345m) fine and to make its software work better with competitors' products.
But an EC spokesman said Microsoft was failing to deliver on its commitments.
"On the basis of market test results, we have serious doubts that Microsoft is complying with the interoperability remedy," EU competition spokesman Jonathan Todd said.
The European Commission said the group had failed on four counts. It was difficult for companies that wanted to licence Microsoft data protocols to get access to its documentation.
Furthermore, companies which wanted to take out a licence would have to pay for an extensive one that also covered items they did not want.
Another limitation was that developers of open source software, which compete with Microsoft in providing software for server computers, could not gain access to the protocols.
But the biggest problem, said Mr Todd, was that: "It would appear that the level of royalties applied would be unjustified."
Interoperability has long been the main charge levelled against the Seattle-based firm, whose Windows operating system powers nine out of 10 personal computers around the world.
Its dominance is much less marked in the market for servers - larger computers which store and distribute data and run networks - and for audio-video software.
In both areas, the EC ruled that Microsoft had made it more difficult for other companies to make their products compatible with Windows PCs - restricting their markets and boosting its own market share.
Under European Union law, companies with dominance in a market face restrictions on their use of that power to leverage market share elsewhere.
Microsoft said it was pleased to have the results of the European Commission's market testing "as it enables us to respond promptly in an appropriate way to work through the issues raised with the Commission".