If you're going to run off the car battery/alternator I would consider using at least part of the laptop's power supply; the DC regulation and smoothing part.
Computer motherboards are quite sensitive and will want a nice steady DC voltage. What you get off a car alternator will be quite rough and ready.
What I would do is take apart the laptop's power adaptor, take out the transformer and connect your power cables from the car battery to the power supply's regulation and smoothing circuits.
This way you will get a clean DC feed for the laptop plus it can use it's own circuitry to charge the laptop's original battery so that when you stop the car, it keeps the computer going without draining the car battery.
That's what I'd do if I was using a laptop.
If you're going to do the Via/Epia/Mini-ITX job, try looking at this install:
http://www.mini-itx.com/projects/ice-unit/Studying electronics occassionally has it's uses.
