European judges will rule tomorrow in a legal battle that could change the way the world's biggest computer software company does business.
Microsoft, whose Windows operating system dominates the market, has been engaged in a fight with European Commission competition regulators for nearly a decade.
Now the landmark case has become a trial of strength which will bolster or seriously curb the Commission's growing anti-monopoly crusade against big companies.
In 2004 Microsoft faced the highest single fine ever imposed by the Commission - $334 million - for allegedly squeezing out market rivals by selling the Windows operating system only with its own Media Player software.
The Commission ordered the company to start selling a version of Windows without Media Player and to share with rivals the necessary communications codes "at reasonable cost" so that they could market their own software to go with Microsoft Windows.
Microsoft did offer Windows without Media Player, but the company claimed that relatively few sales proved consumers wanted the complete Microsoft package.
The Commission threatened even higher fines after complaints from rivals that they still were not getting sufficient Microsoft codes to enable "interoperability" between different computer operating systems. And earlier this year, even as a Microsoft appeal was being considered by judges in Luxembourg, the Commission warned of yet more fines for setting fees too high for access to code information.
The European Court of First Instance - a panel of 13 judges at the EU's second-highest court - has been considering the appeal verdict for 15 months in what is the highest-profile anti-trust case in EU history.
It hinges on to what extent big companies can retain ownership of their technological developments - especially if "bundling" branded software together as a retail package effectively means market domination.
Microsoft says customers want the package, and claim a verdict against the company could stifle innovation.