How do you plan on powering your car pc (inverter, opus, etc)?
I have made an educated guess that you are planning on wiring your car pc using an inverter (please correct me if i am wrong). So what you could do is replace/move the inverter switch up to the front or use a relay to trigger it.
I have included some very basic schematics of how everything would be connected. Basically the carputer will only start when the key is in the ignition with the acc line is powered AND the switch you install is on.
Also I believe you will have to short the contacts the "on" contacts on your motherboard and/or set your bios to "Boot After Power Failure". Or something like that some other member may be able to help with this bit.
Also check the Schematics/Pin Outs/Diagrams Thread for some useful schematics!