Windows 95 design document:

The first major design direction we investigated was a separate UI ("shell")
for beginning users. The design was quickly mocked up in Visual Basic and tested
in the usability lab. (See Figure 4.) While the design tested well, because it
successfully constrained user actions to a very small set, we quickly began to
see the limitations as more users were tested:

1. If just one function a user needed was not supported in the beginner shell,
s/he would have to abandon it (at least temporarily).

2. Assuming that most users would gain experience and want to leave the
beginner shell eventually, the learning they had done would not necessarily
transfer well to the standard shell.

3. The beginner shell was not at all like the programs users would run (word
processors, spreadsheets, etc.).] As a result, users had to learn two ways of
interacting with the computer, which was confusing.

For these reasons and others, we abandoned the idea.
Importantly, because we
used a prototyping tool and tested immediately in the usability lab, we still had plenty of time to investigate other directions. 

http://www.sigchi.org/chi96/proceedings/desbrief/Sullivan/kds_txt.htm

Edit: fixed link.