NightMare

NightMare was a scareware application created by Patrick Evans in 1990.

If the person executes the code "Nightmare" in the Amiga Disk Operating System (DOS), it is executed every 5 minutes in the background. At this point, the screen will cut to an image of a skull from the right side, with his mouth open, teeth covered with blood, and a bullet hole in his back from which blood was leaking. This is accompanied by a resonant screech and occurs for four fifths of a second.

Links
NOTE: The following video contains a screamer!
 * youtube.com/watch?v=ubFGj4lQSyM