as for the carputer, if you have a knowledge of computers, it's pretty cake.
you need a small motherboard (mini-itx, nano-itx, etc) or a normal motherboard if you want to mount it in the trunk or something, some of the smaller ones can be mounted under a seat... etc. you'll also need a 12volt powersupply with auto shutdown on key off, and other typical pc components, as for the screen there are a few choices and i think most people use the liliput
www.mini-itx.com
www.mp3car.com
www.carputers.org
www.gotfrogs.com/carpc.htm
some companies sell the pc for your car, but it's cheaper to build your own...