user35589: I located the two pins that are responsible for powering up the PC. They are connected to the Power On button and one pin has an output of 5 V while the other is the ground.
So far so good, the problem is when I connect the 5 V one to the Arduino VIN and a ground cable from the PC power source to the Arduino GND, instead of powering my board, the PC starts and then promptly stops, as if I pressed the power button and did not release it. 
It seems that electricity passes through the Arduino without powering it. I tested the VIN and GND pins with a normal breadboard power source and they are not broken, as the board starts up with no problem.
Why is the computer starting up instead of powering the Arduino? How can I use my Arduino to start up the PC?
My plan was to power the Arduino with the 5 V pin and send a digitalWrite(somePin, HIGH) for a short amount of time to the ground pin from the PC that would normally be connected to the power button, thus "closing" the circuit like the normal case button would.

user24459: The two pins aren't +5V power and ground.
One of them will be what you think, the other will be a signal wire with a pull up/down.
So if the ground wire really is ground then the other wire is the button input and is pulled up with a resistor, probably in the 10k region.
You can't power an arduino with a ~10k resistor in series with the power, it's just not going to work. But it will pull enough current to make it look like the button is pressed all the time.
What you need to do is:
1) Find out which pin is the signal pin. The easiest way to do this is find some other ground point on the computer (e.g. from one of the HDD power connectors) and measure the voltage from that to the button wires when the button is pressed. If both wires go to 0V then it's the wire that's normally +5 that is the signal, if they go to +5 then it's the wire normally at 0V that's the signal.
2) Fine a real power source. Most PC power supplies have an always on supply that will be at 5V when the PC is off. Use that to power the arduino.
3) Now you know which wire is the signal wire and have a usable power source you can connect the signal wire to one of the arduino pins and drive it to the correct state. Ideally you'd do this via an open collector type interface so that you don't get two different things pulling to very slightly different idle levels.

35589: <p>I located the two pins that are responsible for powering up the PC. They are connected to the <code>Power On</code> button and one pin has an output of <code>5 V</code> while the other is the <code>ground</code>.</p>

<p>So far so good, the problem is when I connect the <code>5 V</code> one to the Arduino <code>VIN</code> and a ground cable from the PC power source to the Arduino <code>GND</code>, instead of powering my board, the PC starts and then promptly stops, as if I pressed the <code>power button</code> and did not release it. </p>

<p>It seems that electricity passes through the Arduino without powering it. I tested the <code>VIN</code> and <code>GND</code> pins with a normal breadboard power source and they are not broken, as the board starts up with no problem.</p>

<p>Why is the computer starting up instead of powering the Arduino? How can I use my Arduino to start up the PC?</p>

<p>My plan was to power the Arduino with the <code>5 V</code> pin and send a <code>digitalWrite(somePin, HIGH)</code> for a short amount of time to the ground pin from the PC that would normally be connected to the power button, thus "closing" the circuit like the normal case button would.</p>
 24459: <p>The two pins aren't +5V power and ground.</p>

<p>One of them will be what you think, the other will be a signal wire with a pull up/down.</p>

<p>So if the ground wire really is ground then the other wire is the button input and is pulled up with a resistor, probably in the 10k region.</p>

<p>You can't power an arduino with a ~10k resistor in series with the power, it's just not going to work. But it will pull enough current to make it look like the button is pressed all the time.</p>

<p>What you need to do is:</p>

<p>1) Find out which pin is the signal pin. The easiest way to do this is find some other ground point on the computer (e.g. from one of the HDD power connectors) and measure the voltage from that to the button wires when the button is pressed. If both wires go to 0V then it's the wire that's normally +5 that is the signal, if they go to +5 then it's the wire normally at 0V that's the signal.</p>

<p>2) Fine a real power source. Most PC power supplies have an always on supply that will be at 5V when the PC is off. Use that to power the arduino.</p>

<p>3) Now you know which wire is the signal wire and have a usable power source you can connect the signal wire to one of the arduino pins and drive it to the correct state. Ideally you'd do this via an open collector type interface so that you don't get two different things pulling to very slightly different idle levels.</p>
