The European Commission will investigate Microsoft once more over claims of monopoly abuse, officials from the supervisory body said today. Two cases are being launched that will address worries that the Windows developer is blocking compatibility with its software, one of which will answer accusations that the company is refusing to supply source code for its Excel and Word programs as well as the .NET programming structure and server products. Without access to key elements of Microsoft's code, rival firms cannot develop software that can guarantee compatibility with these programs or performs as well in a Windows environment, according to the claims.

The investigation will also determine whether Microsoft's Office Open XML (OOXML) format for Office 2007 documents was developed in good faith. Although characterized as easily workable by Microsoft, the format has been criticized by third-party developers who said last year that Microsoft has not taken steps to ensure OOXML will work properly with competitors' products. Challengers have typically promoted the more explicitly cross-platform Open Document Format (ODF).

A second suit is in response to Opera's complaint against Microsoft, which charges the latter of shutting down competition in web browsers by mandating that Internet Explorer be included with every copy of Windows.

These probes into Microsoft represent added difficulties for the Redmond, Washington company, which lost a European appeal in September that sought to overturn a 2004 antitrust ruling. The latter forced Microsoft to ship versions of Windows XP to Europe without Windows Media Player installed after RealNetworks and others complained that they were unable to compete against formats and services offered through the Microsoft jukebox.