Here is some help from Acer, but this applies to all XP systems in general:
http://support.acer.com/acerpanam/desktop/0000/Acer/Veriton9100/Veriton9100faq9.shtml
If your system is in Stand By mode and the keyboard is configured to allow it to bring the system out of Stand By, pressing any key on the keyboard will be the "Wake Up" key - it doesn't have to be the "Wake Up" key - they just put that there so you will think it is cool.
Also, in Device manager, check the same Power Management Properties for your mouse if you want to allow a mouse click to bring the system out of Stand By.
The Wake Up key or the mouse click will only work if the system is in Stand By mode. In Stand By, power is turned off for most things but the motherboard is still "alive" with reduced power and will be able to detect a pressed key or a mouse click.
If you Hibernate or Shut Down that is a different story... the Wake Up key or mouse click will not bring the system out of Hibernate or Shut Down. You are going to have to use the power button to bring the system back to life.