while you CAN use one of those cigarette lighters in your computer, you can completely get rid of the middleman (car plug) altogether.
Like those links show, a cigarette lighter uses ground and 12v, as do PC power supplies. If you got one of these lights, just rip the cigarette lighter part off and use the 2 bare wires. Wire those up to a molex connector, and you're all done. You could wire in a simple switch also if you wanted to ability to turn the light on/off as opposed to it being on whenever the PC is on.