A computer science professor says it's doubtful Microsoft could comply with the anti-trust penalties put forward by nine US states.
Professor Stuart Madnick, of the Massachusetts Institute of Technology, testified in ongoing hearings weighing Microsoft's anti-trust settlement with the Justice Department.
He repeated the firm's argument that removing features like Internet Explorer from the desktop would have unintended consequences.
Professor Madnick said the penalties would "greatly slow, if not end, innovation in Microsoft's major products".
The states want Microsoft to produce a modular version of Windows which more readily incorporates third-party software.
Under questioning, Professor Madnick admitted he could not name another company that melded a web browser into an operating system.
The states claim Microsoft does so to gain market share and to protect an operating system monopoly.
In earlier testimony, California assistant attorney general Tom Greene described Internet Explorer as "a fruit of the poisonous tree of Microsoft's illegal conduct".
This is the fourth time Professor Madnick has worked in a case for Microsoft. He also provided expert testimony in Microsoft's legal case with the European Union and in a class-action suit in California.
Other penalties sought by the states would require Microsoft to disclose technical information to software and hardware developers, and make the company licence its Office business software for use on competing operating systems.