Powering a computer that is intended to run off your 12V battery in your car requires a different type of power supply than the one you would use in your home computer. Computers use DC power and home computers use an AC-DC power supply to convert from your AC home outlets. Since your car uses DC power already, no need to convert. So for a car PC you need a DC-DC power supply. Since this is a very specialized computer that doesn't have the same power requirements as your home pc, you don't need as much power to run it.
After reviewing the various options out there for DC-DC power supplies, it looks like the M2-ATX is a good choice for my project.