Have a read of the following sections of Bill's statement from 2002:
216. In a purely theoretical world, one could imagine developing modest software programs in such a way that any module could be swapped out in favor of a similar module developed by a third party. The replacement module would need to conform identically to the interfaces expected by all of the modules with which it interacts. In the commercial world, it is hard to see what value such replace-ability would provide even if it could be achieved. [Apart from allowing other companies to compete with Microsoft - GD] For Netscape Navigator to suffice as a replacement for Internet Explorer, for example, developers at Netscape would have to devote enormous effort to matching the functions of Internet Explorer and enabling those functions to perform in precisely the same way as Internet Explorer. When they were done, they would have software that is nearly identical to Internet Explorer (a “clone”), providing little or nothing in the way of new value.
217. In addition, if Microsoft were obligated to allow ISVs to clone all the functions of all the “Microsoft Middleware Products” in Windows, Microsoft’s ability to improve Windows would be hampered because the interfaces between modules would necessarily be “frozen” so that third parties could write to them. Given the large number of “Microsoft Middleware Products” in Windows under Section 22.x, the effect would be to freeze large parts of Windows.
The sentence I underlined could have been taken from any book on good programming practice. Was Bill really suggesting that Microsoft doesn't document and adhere to the specified APIs when passing off code requests between Windows and other Microsoft applications? Surely the two development teams don't have access to each others internal documentation? If so, it makes it difficult to continue to claim the existence of the legendary 'Chinese Wall' between Microsoft's OS developers and their application developers. I don't think anyone every took that claim seriously but it's interesting to read Bill testifying to the fact that it technically can't exist.