A program that is described as an operating system because it cannot be exited, it provides a windowing system, and programs can be run under its nominal control.

In other words, like a version of Emacs that takes longer to load and takes the system with it when it crashes and virtually requires a mouse, even for the experienced user.

It is regarded as easy to install, mainly because it is never actually installed. Upgrading the basic OS takes far more than a single kernel compile and reboot; in fact, most software has to be reinstalled afterwards.
Reinstall, however, is something of a misnomer, since Windows does not allow programs to be removed completely and cleanly with any degree of effectiveness.