The following is a list of yet more computer viruses. Your system works fine, but it complains loudly about foreign software. Makes its presence known, but doesn't do anything. Secretly, you wish it would. Suddenly appears from nowhere, promises to upgrade all your applications, gains control of your computer, then totally corrupts your entire system for four years.
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. Invades your system in places where no virus has gone before. The computer locks up, and the screen splits in half with the same message appearing on each side of the screen.