The European Commission has opened two new antitrust investigations of Microsoft's activities.

The first case is in response to a complaint from the European Committee for Interoperable Systems, a Brussels-based trade group of which Opera Software is a member, and concerns the interoperability of Windows with other software, the Commission said Monday.

The second investigation is looking into Microsoft's tactic of bundling software products with its Windows operating system. This follows a complaint to the Commission by Opera, a Norwegian browser developer.

Both issues featured in the Commission's landmark March 2004 antitrust decision against Microsoft, which the company unsuccessfully challenged in court.