The European Commission opened a new antitrust probe against Microsoft on Monday into whether it unfairly tied its web browser to the Windows operating system and made it harder for rival software to work with Windows.

The new investigation follows a landmark European Union court decision last September that Microsoft could not exclude rivals by tying its products to its near-monopoly Windows product and must allow rival software to interoperate smoothly.

One complaint was brought by Opera, a Norwegian maker of a web browser, which competes with Internet Explorer.

The second was brought by an industry group, the European Committee for Interoperable Systems, which said Microsoft did not disclose enough interoperability information for a range of products.

"This initiation of proceedings does not imply that the Commission has proof of an infringement. It only signifies that the Commission will further investigate the case as a matter of priority," the Commission said.