You can find the hot/positive and ground leads on the switch on the PC, remove them from the switch and simply connect them together either by soldering or using crimp-on connectors. Make sure you do any work on the PC with the power plug disconnected from the PC. By connecting the hot and ground leads together the PC will turn on when power is applied to the power supply.
To turn on/off the PC you can either install a 30A SPST(single-pull, single-throw) switch somewhere between the power source and the PC. Remember to install either 25A or 30A fuse on the power connections on both the hot(positive) and ground leads close to the power source. To get the PC to turn on/off with the ignition you can run a 14 or 12 gauge wire from an empty ignition-switched fuse slot in your interior fuse box to a 30A relay like the one Radio Shack sells, Cat.#: 275-226. That relay has four contacts. You connect these connections to its own contact on the relay: the hot power lead coming from ...say your battery, the wire from the ignition-switched fuse slot, a ground wire, the hot lead going to the PC. You would ground the PC to either chassis or the ground/negative on the battery.
The relay will sense the voltage on the ignition-switched contact when you turn the key and it will then close the circuit to the main hot power lead thereby applying power to your PC. When you turn your key off the relay will sense the loss of voltage on the ignition-switched contact and open the main hot lead going from your battery to the PC.
I strongly advise to have someone experienced do this work or assist you if you've never done it before.