Comment by mike_hearn

It's easy to forget in these discussions that Microsoft didn't have infinity resources available when writing Windows, and often the dodgy things apps were doing only became clear quite late in the project as app compatibility testing ramped up. Additionally, they had to work with the apps and history they had, they couldn't make apps work differently.

You say, oh, obviously you just should redirect writes to a shadow layer or something (and later Windows can do that), but at the time they faced the rather large problem that there is no formal concept of an installer or package in Windows. An installer is just an ordinary program and the OS has no app identity available. So, how do you know when to activate this redirection, and what is the key identifying the layer to which redirects happen, and how do you handle the case where some writes are upgrades and others are downgrades, etc, and how do you do all that in a short amount of time when shipping (meant literally in those days) will start in just a few months?