lets say when a computer is turned on, its usb port's got power, when it's off, it's usb port doesn't have power.
what do i exactly have to do, to find out whether my computer is on using an arduino? which wires do i connect to the arduino ports (and to what type ports) and how do i program it to find out whether it's got power or not?
thank you 
mouthfulofchocolate:
lets say when a computer is turned on, its usb port's got power, when it's off, it's usb port doesn't have power.
what do i exactly have to do, to find out whether my computer is on using an arduino? which wires do i connect to the arduino ports (and to what type ports) and how do i program it to find out whether it's got power or not?
thank you 
power it with the USB port?
mouthfulofchocolate:
what do i exactly have to do, to find out whether my computer is on using an arduino?
You could buy one of these USB Mini-B Breakout and a USB lead,
then get one of these: Opto-isolator Breakout
Connect GND, 5V from the USB board to GND, IN1 on the opto board.
Then connect HVGND to Arduino GND and, HV to +5V and OUT1 to an input pin on your Arduino.
Then look at the Button Tutorial
Turning on and off your computer will be like pressing a button.
Yours,
TonyWilk
what do i exactly have to do, to find out whether my computer is on using an arduino?
How is the Arduino powered ?