The cheapest is to use a spare (or one borrowed from another computer) ATX power supply to power the motherboard. Same goes for CD-ROM. A spare drive or one borrowed from another computer. You could also install from a USB thumb drive. There are guides online for that. I've installed Linux and Windows from USB thumb drives. Linux was easy, Windows XP was a pain to prepare the thumb drive. I hear Vista and Win 7 are easier to install from thumb drives.
Yet another option is to take the hard drive out of the car PC, and put it into a desktop (or laptop) and install from there, then put the drive back into the car PC.