The first is to explore whether Microsoft's bundling of its own internet browser, Internet Explorer, with its operating system, Windows, deprives consumers of choice of which browser they want to use to navigate the internet.

The investigation comes in response to a complaint from Norwegian alternative internet browser firm Opera. However, the probe will look not just at the browser bundled with Windows, but allegations that a whole range of products have been unlawfully tied to sales of Microsoft's dominant operating system.

"Browsers are the gateway to the internet. Microsoft seeks to control this gateway," said Thomas Vinje, legal counsel for the European Committee for Interoperable Systems (ECIS), of which Opera is a member.

The second investigation is to look at the 'interoperability' of Microsoft's products with competitors' software, referring to how easy it is for competitors to run software on Microsoft systems.

It follows on from a direct complaint by ECIS, which is an alliance of technology firms promoting interoperability that also includes many rivals of Microsoft such as Adobe, Nokia and Sun.

ECIS had complained that Microsoft illegally refused to disclose information about the 'nuts and bolts' – or interoperability information - of a broad range of its products. By not letting rivals have access to this interoperability information, it is alleged that Microsoft makes it much harder for their software to work more smoothly with its own.

Specifically, the commission is to investigate Microsoft's Office suite of software, a number of its server products, and its .NET Framework. The commission will also look into whether Microsoft's new file format, Office Open XML, as implemented in Office, is sufficiently interoperable.

Microsoft issued a statement saying it would co-operate with the commission's inquiries.

The two new investigations come just months after Microsoft settled another long-running antitrust battle with the commission over its bundling of the Windows Media Player, its video and audio playing software, with its operating system.

In October 2007, the firm finally complied with a commission ruling of 2004 after having lost an appeal and being fined some €500 million.