Welcome newbie!
A good place to start researching your project:
http://www.mp3car.com/vbulletin/newbie/78795-new-carpcs-dont-know-where-start.html
More advanced reading for you:
http://www.mp3car.com/vbulletin/faq-emporium/
And I disagree with 2k1Toaster... The simplest way of powering a laptop would be a car power adapter for the specific make/model of laptop. But an inverter certainly is a simple solution!