Your system works fine, but complains loudly about foreign software. Makes its presence known, but doesn't do anything.
Files disappear, only to mysteriously reappear a year later in another directory.
You know it's guilty of trashing your system, but you just can't prove it. All files reported as the same size. This virus doesn't horse around. Activates every component in your system just before the whole thing quits. Nothing works, but all your diagnostic software says everything is fine.