Is Windows a virus?
No, Windows is not a virus. Here's what viruses do:

They replicate quickly. Okay, Windows does that.

Viruses use up valuable system resources, slowing down the system as they do so.
Windows does that too.

Viruses will, from time to time, trash your hard disk.
Yes, Windows does that.

Viruses are usually carried, unknown to the user,
along with valuable programs and systems
Alright... Windows does that, too

Viruses will occasionally make the user suspect
their system is too slow and the user will buy new hardware
Yup, that's with Windows too

Until now it seems Windows is a virus but there are fundamental differences:

Viruses are well supported by their authors, are
running on most systems, their program code is
fast, compact and efficient and
they tend to become more sophisticated as they mature

So, Windows is not a virus