did you try "car pc"
anyway, it's possible, but hard to do. The first thing you need is a transformer to get the 12vDC to 120vAC...that is, if you wish to put a normal PC in a car. You could probably get a laptop working with little extra effort....
Anyway, it has been done before...the main issues are heat and power. I don't know if an alternator can power the car, it's auxilary components (lights, stereo, computer, etc.), AND a PC at the same time....but I do know that people have managed to do it. The next issues are input and output. A lot of people use just a numpad for their comptuter, in order to use winamp. If you wish to have a fully functional computer, then you won't be able to do this. And as for display, those winamp computers usually use a small dash-mounted LCD....6" is probably a good size for that. But again, if you want a fully functional computer, then you will have to go bigger....but definately you will want a flat-panel LCD.
I'm sure someone will come along and reply....it has been done, it's just a matter of waiting for those people who have done it to read and reply, hehe.