First, if you aren't up to building your own, check these out -
http://www.xenarc.com/product/index-pc.html
Yes, many people have built car PCs. All of the ones I see have used standard PC equipment. Mini ATX motherboards, low profile power supplies, all in custom cases that are made to fit where you want, usually made of MDF with a fan or two. Hard drives are very shockproof, they're designed to to survive being dropped on the floor.
To power the PC all you will need is a DC to AC inverter. PC video cables are shielded, but the power signal is filtered through the inverter so you shouldn't have to worry about noise from the alternator.
Threaded Mode