use an inveter, connect it to d d car battery n fix a three pin socket gang on d dashboard.
this way u can use any electronic instrument tht uv never evn dreamt of, while in d car...by just connecting it into d plug, sub-laptops r d best thing to connect to d car...jst remembr to keep d car in motion so dat the battery doesnt die down..
Using a power inverter as mentioned would be highly unificent way of powering a computer for long time use, I would recommend using a Mini-ITX Board with a pico psu then the PC can be simply power by 12V and if you wanted to use it at home they come with an adaptor, Mini ITX PC's are also very small which is good in a car.
The cheapest way would probably to use a good old power inverter and simply plugging the computer and monitor in it...
Better would be to use a laptop thru an inverter as their power consumption is a lot more controlled and efficient than a desktop.
there aqre dc dc power spplies or you can use ac inverter and the existing power supply
i recommend to switch down video card etc and use better quality power supply to save energy (the battery of the car)
The power supply to the computer could be replaced with a voltage and amperage control circuit, but the monitor might have to have an inverter for AC power.