"We believe the inclusion of the (IE) browser into the operating system benefits consumers, and that consumers and PC manufacturers are already free to choose to use any browsers they wish," Microsoft said in a statement.

"Internet Explorer has been an integral part of the Windows operating system for over a decade," the company said, "and supports a wide range of Web standards."

For Consumer Tired of 'Monopolist'

Opera filed a complaint Thursday with the Commission, claiming that Microsoft had engaged in anticompetitive behavior by bundling Internet Explorer with Windows, putting other browser makers at a disadvantage.

Opera also claimed that Microsoft's weak support for Web standards puts more compliant browsers at a disadvantage because Web developers will focus on the browser with the greatest market share.

"We are filing this complaint on behalf of all consumers who are tired of having a monopolist make choices for them," said Jon von Tetzchner, CEO of Opera. "In addition to promoting the free choice of individual consumers, we are a champion of open Web standards and cross-platform innovation."

Microsoft "only recently begun to offer some of the innovative features that other browsers have offered for years," he added.

Earlier Decision Provides Context

The European Commission acted against Microsoft 2004, but the remedy the Commission came up with was seen as ineffective. The issue at the time was whether Microsoft's bundling of Windows Media Player was anticompetitive. The EC ruled that it was and ordered Microsoft to offer a version of Windows free of the Media Player.

The problem was that the two versions of Windows cost the same, few PC makers opted to preinstall the version without Windows Media Player, and competitors continued to be locked out of Windows PCs.

This time around, Opera is calling not for an IE-free version of Windows to compete with a full version, but for the EC to require Microsoft to preload several competing browsers, including Opera, as part of Windows.

"In our minds, the best solution would be one version of Windows with a must-carry type of provision," said Jason Hoida, Opera's deputy general counsel.

Firefox Success an Issue

The complaint largely mirrors the antitrust action brought against Microsoft in the 1990s after it started bundling IE with Windows. The difference this time around is the success of Firefox, the open-source browser from the Mozilla Foundation, and indeed the explosion of the open-source movement. Firefox has something on the order of 20 percent of market share, which may be seen as proof that bundling of Internet Explorer does not shut out competitors.

But Opera maintains that Firefox's success is too limited and its trend line might be fading. "Firefox has attracted a lot of users, especially in the open-source world, but it has not been climbing consistently," said Hakon Wium Lie, Opera's chief technology officer.

Because the EC already has judged Microsoft a monopolist, it might fashion a harsher remedy this time around. "If there's a replay of what led up to the 2004 ruling, and Microsoft takes similar positions regarding bundling, then I wouldn't be surprised if the Commission comes down harder now," said Chris Norall, a partner in the Brussels office of the law firm Morrison & Foerster.