Beep

Beep is a prank program that will cause a person's computer to emit a loud, high-pitched beep through the motherboard (which means that even if they turn their desk speakers off, it'll still beep). The beep cycle (beeping one second out of every 1.5 seconds) continues until the computer is restarted, at which point the prank is disabled.

If you want to make it start up with Windows, either add it to the startup folder or to HKLM\Software\Microsoft\Windows\CurrentVersion\Run.