According to the agreement that Microsoft signed in December 2009, it cannot set conditions that forbid OEMs from installing other browsers on their PCs. Nor can the company retaliate against an OEM that distributes third-party browsers like Chrome, Firefox, Opera or Safari.
The fourth and final accusation claims that Microsoft does not display the browser choice screen on devices running Windows RT.
The browser choice ballot was the centerpiece of the 2009 settlement, which required Microsoft to give users easier access to rivals' Web browsers.
According to the agreement, which expires at the end of 2014, the browser choice screen must appear in all copies of Windows, including Windows 8. Previously, the commission had declined to say whether it believed Windows RT, the offshoot designed for ARM processor-powered tablets, was covered by the deal.
It appears that EU regulators are now prepared to ponder exactly that.
Although the commission refused to name the parties that reported the alleged violations, Mozilla has been vocal about the browser choice screen and Windows RT. In a May blog post, Mozilla's chief counsel, Harvey Anderson, argued that Windows RT should be held to the same requirements as Windows 8.
"If Windows on ARM is simply another version of Windows on new hardware, it also runs afoul of the EC browser choice commitments," Anderson said at the time.
But a browser choice screen in Windows RT may be hard to populate. No other browser maker has publicly committed to develop for the new operating system, although Google and Mozilla are working on Windows 8 versions of Chrome and Firefox that will include both desktop and Metro versions. The Metro-style Chrome and Firefox browsers should also run on Windows RT, albeit slower than IE10 if Microsoft is not forced to unblock access to Windows RT's APIs.
Microsoft's latest confrontation with the commission is reminiscent of the one that preceded Windows 7's launch in 2009.
In June 2009, near the end of Windows 7's development, Microsoft said it would ship the operating system without Internet Explorer 8 (IE8) to European customers to sidestep the commission pressure and keep Windows 7's launch on track.
For a time, Microsoft stuck to that plan -- it was going to call the European edition Windows 7 E -- but only weeks later it caved and agreed to a browser choice screen.
Windows 8 is much closer to completion than Windows 7 was three years ago when the commission pushed for a browser ballot. It would be very difficult, if not impossible, for Microsoft to modify Windows 8, Windows RT or both, and still make the Oct. 26 on-sale date for the former.
Microsoft has declined to comment on the commission's investigation beyond the statement it issued Tuesday.
Gregg Keizer covers Microsoft, security issues, Apple, Web browsers and general technology breaking news for Computerworld. Follow Gregg on Twitter at @gkeizer, on Google+ or subscribe to Gregg's RSS feed. His email address is firstname.lastname@example.org.
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