1. They replicate quickly - okay, Windows does that.
2. Viruses use up valuable system resources, slowing down
the system as they do so - okay, Windows does that.
3. Viruses will, from time to time, trash your hard disk -
okay, Windows does that too.
4. Viruses are usually carried, unknown to the user, along
with valuable programs and systems. Sigh... Windows does
that, too.
5. Viruses will occasionally make the user suspect their
system is too slow (see 2) and the user will buy new
hardware. Yup, Windows does that, 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.
It's a bug.