Your system works fine, but it complains loudly about foreign software. Makes it's presence known, but doesn't do anything. Secretly, you wish it would.
Files disappear, only to reappear mysteriously a year later, in another directory.
You know it's guilty of trashing your system, but you just can't prove it. Could be virulent, but it's been around too long to be much of a threat. All files are reported as the same size. This revolutionary virus does not horse around. Activates every component in your system, just before the whole thing quits.