Before I got rid of my Honda Civic (absolute dog of a car, worst £1800 I've ever spent), I was converting an old laptop be become a carputer. I had a cable going from the laptop to the AUX port on the CD player head unit and I played music through it. I had intended to put the laptop under the passenger seat and wire the audio round the back of the head unit, and connect a touch screen to the laptop so I could control it like that. It would deffinately be a talking point when driving somewhere!
Also, a man called Jason Bradbury build a Head Up Display which displayed a information from a laptop while driving. Instructions on how he done it are on his website.