The US states suing for tougher anti-trust measures have attempted to show that Microsoft uses Windows to thwart rivals.
The States' lawyers allege successive versions of the operating system have pushed third-party services to the periphery.
The questioning came as the Microsoft's corporate vice president of Windows Client Group replaced Mr Bill Gates in the witness box.
In his testimony Mr Chris Jones argued the proposed punishments would diminish Microsoft's ability to produce software that's easy to use.
But he admitted many decisions the company makes about the desktop's appearance are driven by business motives.
States' lawyer Mr Kevin Hodges said each Windows upgrade, from 95 to XP, has featured Microsoft services more prominently.
Mr Jones said computer manufacturers are free to place icons for third party services next to their own icons if they wish. But Mr Hodges pointed out that they have to Microsoft pay to do so.
The two also battled over a definition for Internet Explorer and whether it could be removed as the states wish.
The states want to let computer manufacturers remove Internet Explorer and other features of Windows and substitute competing software.
That, the states say, would reduce Microsoft's advantage, give consumers more choice and let software developers make different kinds of programs.