You'll need another Mac and a FireWire cable. Right after you press the power button, press the T key on the keyboard. You'll then see a FireWire logo bounce around the screen. At this point, connect the two Macs with a FireWire cable and the iMac will appear on the other's desktop as an external hard drive. You may need to use a wired keyboard for this. Also, if both Macs have Thunderbolt, a Thunderbolt cable will work with the same procedure.