However, the European Commission noted that an agreement it struck with Microsoft in 2009 to regulate its behavior applied only to "client PCs," and it wasn't immediately clear if that incudes tablets, one of the main targets for Windows RT.

Mozilla protested last week that Windows RT, the version of Windows 8 for devices that use ARM processors, restricts choice and an agreement Microsoft struck with the Commission to resolve an antitrust probe of the company.

Windows RT grants full access only to Internet Explorer (IE) and effectively blocks other browsers from accessing important functions, according to Mozilla. Its complaint was backed by Google, which makes the Chrome browser.

"The Commission is aware of these allegations and will remain vigilant that Microsoft fully complies with its commitments under the Commission's 2009 decision on browsers," Antoine Colombani, the Commission's spokesman for competition, said via email Tuesday.

"This decision applies to Microsoft's Windows operating system for PCs," he added.