They replicate quickly.
OK, Windows does that.

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

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

Viruses are usually carried, unknown to the user, along with valuable programs and systems.
Sigh... 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 recently it seemed that Windows might in fact be a virus, but there are some differences:

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

Therefore, Windows is NOT a virus.