NightMare

NightMare
NightMare is a scareware application created by Patrick Evans in 1990 for Amiga computers. When executed, it will run in the background. Every 5 minutes, the screen will switch to a picture of a skull with its mouth open, blood covering its teeth and a bullet hole on the back of the skull, blood leaking from it. This is accompanied by an echoing shriek and happens for a four-fifth of a second.

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