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 (ECIS), 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.

Microsoft said it would cooperate with the investigations. "We are committed to ensuring that Microsoft is in full compliance with European law and our obligations as established by the European Court of First Instance in its September 2007 ruling," the company said in a statement.

Both new probes build on the findings of the 2004 ruling, which were upheld last September by Europe's second-highest court, the Court of First Instance (CFI). Microsoft decided not to appeal the CFI decision, so the precedent value of the 2004 ruling remains intact.

The first of the new probes will examine whether Microsoft withheld information from companies that wanted to make products compatible with its software. This includes word processing, spreadsheet and office management tools contained in Microsoft's Office suite of software applications. It also includes some server products and Microsoft's .Net Internet software framework.

ECIS filed a complaint to the commission in 2006, arguing that Microsoft's failure to share interoperability information amounted to an abuse of its dominant position in the market. ECIS members include IBM, Nokia, Sun Microsystems, RealNetworks and Oracle.

In addition to ECIS' complaint, the commission said it will look at whether Microsoft's open format for archived documents—Office Open XML—"is sufficiently interoperable with competitors' products."

"ECIS welcomes the Commission's announcement as a necessary step towards ensuring Microsoft's compliance with competition rules," the group said in a statement Monday.

"It is regrettable that despite the judgment of September 2007, Microsoft continues to use its desktop monopolies to restrict competition," said Thomas Vinje, ECIS' spokesman.

The second probe, sparked last month by Opera's complaint, will look at whether Microsoft illegally bundles the Internet Explorer browser for free with Windows.

Opera wants the commission to strip Explorer out of Windows or carry alternative browsers. It claims that new proprietary technologies in Explorer hold other browsers such as Opera back, by not following open Internet standards.

The commission is also looking into whether Microsoft has illegally packaged desktop search and Windows Live into Vista, the latest version of Windows.