The European Commission launched a new antitrust investigation into Microsoft on Monday, to determine if the software company broke EU competition laws to give its web browser Explorer a leg up over its rivals. The complaint takes aim at one of the software giant’s most popular products, which has an overwhelming share of the market. (The EU has also launched an investigation at the same time into whether or not Microsoft (NSDQ: MSFT) abused its dominant position over its Office and Outlook products.) Reuters reports the Commission is asking if Microsoft is using the same tactics to deter competitors as it did with its Windows Media Player. That case—a six-year investigation—ultimately cost Microsoft 778 million euros ($680 million at the time) in fines.

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Norwegian browser firm Opera complained about Explorer in December, spurred by the success of the Media Player case. It alleges that Microsoft has tied Explorer to Windows and is hindering interoperability by not following accepted web standards. The company argues that by not adhering to standards, the software giant shifts the onus on web developers to create compatible internet sites. With Explorer the dominant browser on the market, developers overwhelmingly create sites to work with Microsoft’s web browser, shutting out its rivals.