Microsoft's offer to allow rivals access to its software blueprint may not be enough to prevent European Union action, the EU has warned.
Microsoft may have to do more to satisfy the EU's demands
The EU has threatened to fine the group 2m euros (£1.4m; $2.4m) a day if it fails to supply adequate information about its server programs.
Brussels will decide whether the offer is enough when it receives full details, a EU spokesman says.
The news came as Microsoft posted a 5% rise in quarterly profits to $3.65bn.
The group said the launch of the Xbox 360 games console and the new SQL Server 2005 corporate database software program had helped boost its profits during the period.
Demand for the products helped push net earnings up to 34 cents a share from 32 cents last year.
Microsoft is currently racing to meet a 15 February deadline to comply with EU demands to open up its software to rivals.
Its offer to open up its software blueprints "underscored its commitment" to meet the European Commission's demands, Horacio Gutierrez associate general counsel for Microsoft Europe said in a statement.
However, Brussels has warned the offer may not go far enough.
"It would be premature to conclude that offering access to source codes would necessarily resolve the problem of compliance," said EU anti-trust spokesman Jonathan Todd.
Competition Commissioner Neelie Kroes added that users needed more than just the code - they also need comprehensive instructions that would allow them to develop software compatible with Windows systems.
"Normally speaking, the source code is not the ultimate documentation of anything," she said.
"[This is] precisely the reason why programmers are required to provide comprehensive documentation to go along with their source code."
She added that she had little to go on regarding Microsoft's proposal as so far she had only received a letter and a press release.
"When we have the full details I will come back."
Earlier, Microsoft's legal chief Brad Smith insisted the group was "putting our most valuable intellectual property on the table so we can put technical compliance issues to rest and move forward with a serious discussion about the substance of the case".
The compliance deadline was set in December, when the European Commission said Microsoft's offer of 12,000 pages of documentation and 500 hours of free technical support was not adequate.
The landmark 2004 ruling said the world's biggest software company was guilty of abusing its position and hit Microsoft with a record 497m euros fine, telling it to open up its operating systems.