Pressure increased on Microsoft over US antitrust legislation as the Department of Justice asked Netscape Communications to supply it with documents relating to browsers.
The investigation will focus on whether Internet Explorer is unfairly competing against Netscape and other browser companies by effectively giving the software away, so growing market share.
Other concerns that the DoJ is understood to have are reports that Microsoft will include components of its Office suite in its browser software, effectively locking users into IE.
The investigation follows a letter Netscape sent to the DoJ last August alleging that Microsoft had put unfair pressure on its OEMs to use IE rather than the Netscape Navigator by threatening existing Windows licensing agreements. That is a claim that Microsoft denies.
This investigation is separate from another, prompted this week after IBM and other OEMs complained about clauses in the licensing agreement that prevent them inserting code during the Windows boot process. The fresh OEM agreement goes further than previously thought, it has emerged.
According to a source at another major hardware vendor, the agreement also prevents them from putting bitmaps of their company logos as a background to Windows, formerly a common practice for firms like Compaq and IBM.
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