E.U. antitrust regulators struck a deal with Microsoft , ending an antitrust investigation of its dominant position in the browser market.

Since then, Microsoft has been required to display a ballot screen in Windows with links to other browsers including Google's Chrome and Mozilla's Firefox.

The Commission wouldn't say Tuesday if it believes Microsoft is violating the terms of the agreement. However, Colombani emphasized that the 2009 decision "doesn't go any further" than PCs.

Windows RT is intended primarily for tablet devices, but the OS will also be able to run on PCs that use ARM processors, to Microsoft.

Microsoft may have decided it's debatable whether the 2009 browser agreement applies to Windows RT, said Roger Kay, an analyst at Endpoint Technologies. That might be a risky bet, he added. "If they are wrong then they basically will get their wrist slapped," Kay said.