The proceedings at the Court of First Instance in Luxembourg are expected to conclude this Friday (April 28), but the 13 judges on the panel will not rule on the appeal for another year.
The European Commission two years ago found Microsoft guilty of monopolistic practices, fined it $612 million and ordered it to change its behavior. The commission ordered the software giant to sell a version of its Windows operating system unbundled from its Media Player software and to divulge information on its operating system to makers of rival products.
Microsoft will argue in its appeal that the trial is about companies' IP rights. "At issue are whether companies can improve their products by developing new features and whether a successful company must hand over its valuable intellectual property to competitors," Microsoft said in a statement ahead of the trial.
The European Commission has stated that it is confident its 2004 ruling will stand.
Leading up to the hearing this week, the tension between Microsoft and the European body had intensified. In December the EC threatened to slap a daily fine of up to 2 million euros (about $2.5 million) a day on the software giant.
Microsoft also has antitrust troubles elsewhere in the world.
In February, after a successful antitrust prosecution in South Korea, that country's trade commission ordered the software giant to offer two new Windows editions in the Korean market. The commission also fined Microsoft 32.5 billion won (about $33.3 million).
Microsoft filed an appeal in that case in late March.