These instructions are for use with DC-DC power supplies.
If you want to measure power from an AC to DC supply, you can use the same instructions, but it is extremely dangerous to fool around AC power if you don't know what you are doing. Don't do it if you feel the least bit uncomfortable with the idea. One exception to my instructions if you use AC is to not bother taking a voltage reading and just use 120V as your voltage.
If you really want the easiest way to measure the computer systems power here is what you do. Take your multimeter and change it to the ammeter. Most mulitmeters have a 10amp ammeter setting, that is the one you want. Also, most multimeters have a different socket for the 10amp multimeter connection, make sure you switch your lead to that socket before you start measuring current or else you will blow the multimeter fuse.
Ok, now that you have your multimeter set up properly, dissconnect the red (positive) wire that leads to your computer power supply. Then attach your multimeter in series with the two lines. That is, connect one lead from your multimeter to the wire coming out of your power supply, and the other lead to the wire that goes to your battery.
Once you have set this up as described, turn on your computer and when the computer is running full power, record the amps that the multimeter displays (it might display a negative number, this doesnt matter, just treat it as a positive number). I would assume you see around 7 amps or so at full computer power. After that, disconnect the multimeter and reconnect the power supply as it was. Next, set your multimeter to read DC voltage and with your computer turned on, and running full power, test the voltage at the input to your power supply by attaching the black lead of your mulitmeter on the negative wire, and the red lead of you multimeter on the positive wire. I would guess you read about 12.5 volts. Record the voltage that you read.
Next for some simple calculations.
Take your amps number, multiply it by your volts number and you have Watts (power).
Volts x Amps = Watts.
Hope that helped.