you can use an inteverter. That takes the 12 volt dc power from you car and changes it to AC power which a normal power supply can handle. Ironically the computer power supply turns it into dc power that your computer needs. So if you get an inteverter, you can use the normal desktop setup you have now
on the other hand, you can get a dc-dc power supply that changes the 12volt power and pretty much splits it up into the power needs that your motherboard requires.
dc-dc PSU is what everyone will recommend. It'll cost more then an inteverter, but there are a lot of postives using the dc-dc option. The big two is that its more efficent and also quiet. The other big one is inverters can bring in line noise into your setup that you have to fix.