A EU competition panel has unanimously backed a draft plan of action which could see US software giant Microsoft hit with fines and penalties.
Microsoft and Brussels have failed to agree on the issue
The EU advisory panel of national antitrust experts endorsed a decision drawn up by the European Commission.
Reports say Microsoft will be forced to share proprietary details with rivals and provide a second, stripped-down version of its Windows system.
The decision paves the way for a final announcement later in March.
The advisory panel's latter demand would allow rival brands of audiovisual software to be built into Microsoft's Windows operating system.
Microsoft would have to offer consumers a choice of buying Windows with or without the company's own AV software.
The panel is also expected to endorse a fine of hundreds of millions of euros against Microsoft, although not until a second meeting scheduled for 22 March.
The European Commission is due to announce its final decision on the issue on 24 March.
The two sides have tried and failed to reach agreement on the issue, which has dragged on since the early 1990s.
The European Commission has long been concerned about the dominance of Microsoft's Windows operating system.
The current case argues that this dominance has prevented rival versions of multimedia software gaining a foothold, and that Microsoft has given too little information about how Windows works.
Brussels reportedly intends to give Microsoft a deadline to get its house in order or face additional action.
Commission officials have worked and re-worked their draft to stand up to court scrutiny, since Microsoft is almost certain to appeal against any adverse decision.