Testimony from Mr Paul Maritz, the first Microsoft senior executive to defend the software giant against antitrust charges filed by the US government, is expected to be heard today.

Mr Maritz oversees the development of most of the company's computer and server software, including the Windows operating system at the heart of the lawsuit filed against Microsoft by the US Justice Department. His name repeatedly has come up in Microsoft electronic mail that the government cited in its case against the company during the first part of the trial, which began four months ago.

In written testimony, Mr Maritz said that the rapid pace of innovation in the computer industry and the tough competition meant that no company, Microsoft included, has a guaranteed edge.

"Most existing software products will be rendered obsolete in three years if not sooner," Mr Maritz said. He cited the case of IBM, which failed to respond to the need for personal computers, opening the door for rivals such as Compaq, Dell and Gateway 2000.

He said that a similar fate could await Microsoft if it fails to adapt to its clients' needs rapidly for Internet functions.

The US Justice Department has accused Microsoft of engaging in anti-competitive and predatory practices aimed at extending its dominance of the operating systems market to Internet-related products, notably web browsers.

The government in particular charged that Microsoft had "tied" its Explorer browser to its Windows operating system - now in use in 90 per cent of all personal computers worldwide - to stamp out competition from rival software firms.

Microsoft argues that the browser was designed as an additional function of Windows in response to consumer demands for integrated technology.

Mr Maritz referred to Windows competitors such as Linux, versions of Unix, Apple's Mac OS and IBM's OS/2 Warp.

Linux is increasingly challenging Windows, he said, citing as other threats the navigation software on the Netscape Web and the universal programming language Java.

The growing array of electronic equipment, simplified machines at lower prices allowing access to information via the Internet or other networks, and the use of "intelligent TVs" pose additional threats to Microsoft's dominance.