Asus PU-DLS - Manual (Page 56)

3-2 Chapter 3: Powering up
3.2 Powering off the computer
You must first exit the operating system and shut down the system before
switching off the power. For ATX power supplies, you can press the ATX
power switch after exiting or shutting down the operating system. If you
use Windows 2000/XP, click the Start button, click Shut Down, then click
the OK button to shut down the computer. The power supply should turn
off after Windows shuts down.
The message “You can now safely turn off your computer” does not
appear when shutting down with ATX power supplies.