Software
No one piece of software does everything you might want in a Car PC — the developer would have to be wildly creative to have a priori thought of everything anyone might want a Car PC to do. Unless you’re building a Car PC to do a small number of very specific tasks, you’ll want the software to be a collection of programs tied together by a configurable, flexible menu and program launcher. Done that way, each function in your Car PC — such as DVD playback, navigation, and others — can have unique software, software that you can update or replace at will.
Whatever software architecture you use, however, the way the system interacts with the driver is crucial. The fact that we’ve seen people on freeways in Los Angeles and elsewhere driving the speed limit (and more) while reading, putting on makeup, and worse doesn’t change the fact that it’s irresponsible to take as much attention off the road (and your hands off the wheel) as is required to operate a keyboard or mouse. If the software is well designed with large easily read buttons, touch screens solve much of the problem by creating an interface similar to that used successfully for controls in cars for decades.