In 2009, the EU's executive arm, the European Commission, told Microsoft it must offer users a choice among browsers to ensure the company wasn't using its dominant market position to foist its Internet Explorer browser on users.
Microsoft created a "choice screen" but in July conceded that a technical error meant that millions of users in the EU may not have seen it between February 2009 and July 2012.
The Commission formalized its complaint Wednesday, although the investigation continues.
Microsoft could face a fine of up to 10 percent of its annual revenue if found in breach of antitrust law.
(Copyright 2012 by The Associated Press. All Rights Reserved.)