Seriously, it's not very hard. Then again computers is also my specialty. Basically, here's what you need.
For the screen, go with a touch screen, you'll thank yourself later, don't bother with getting just a DVD player. Remember, RCA inputs for video is BAD.
For the PC this is where it gets a little complicated. The BEST way of doing this is to buy a 12V PSU. I can talk to my friend and see what he used. But basically it used the same idea as an amp. Car turns on, deck sends signal on remote line, PC turns on. Car turns off, it sends remote signal, windows interprets it as a shutdown command, and the PC turns off. Easy as pie.
As for the PC itself, obviously the smaller the better, if you have extra cash go with a mini-itx system.
www.mini-itx.com. If you are trying to go with a smaller budget go with a mATX board. They're a couple inches bigger than the mini-itx but about 1/3 of the price with more features.
You don't need a special CPU, Video Card, RAM or any of those. They're just standard PC components. Basically price wise, you're looking at about $40 for a case, $100 for a GOOD motherboard, $90 for a GOOD cpu, $45 for 1gb of ram, $80 for a 320gb hd, $70-$80 for the PSU, $130 for windows, $140 for Streets and Trips w/GPS, $250-$300 for the screen, and $30 for an amp wiring kit. That's if you go the mATX way.
Building it yourself WILL be cheaper than buying a premade.