1. Viruses replicate quickly.
Windows does this.
2. Viruses use up valuable system resources, slowing down the system as they do so.
Windows does this.
3. Viruses will, from time to time, trash your hard disk.
Windows does this.
4. Viruses are usually carried, unkown to the user, along with valuable programs and systems.
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.
Same with Windows, yet again.
Maybe Windows really is a virus.
Nope! There is a difference!
Viruses are well supported by their authors, are frequently updated, and tend to become more sophisticated as they mature. So there! Windows is not a virus.