rite i get 2 meet quite a few people in work (halfords) and i met this computer tech who has a mind blowing computer so i found out quite a bit about computers. well laptops are out of the question for a start.
generally tower pc's al have reducers to take the voltage down to generally 12V tho this varies a wee bit between PC's. a lot of hard drives have for some reasons two different voltage feed. but i know you can get a direct replacement for the reducer inside the computers so it can run of 12V directly and regulate this to avoid blowing it up, lol. as for where you can get it from im not too sure. im going to see about building a carputer myself as soon as i get the nessecary bits (exhaust/new wheels) 1st. i was just going to get a cheap tower pc, upgrade any components necessary (eg. ram) and get this guy to get me the nessecary part for running off the correct voltage. i see him every month or so and unfortuantly i seen him this weekend so i may not see him till around christmas time but if i do ill get details of the part required and pass it on to ya
Do you actually know WTF your talking about? By the sounds of it your 'mate' doesn't either so I guess its not your fault. A PC power supply (PSU) take mains voltage (230-250v AC) and converts it into seperate voltage 'rails' 12V 5V 3.3V. A hard drive does NOT run of 2 voltage feeds

. It runs off a 12V rail and scales the voltage down for its own requirements usually 5V.
As for original post it is actually very simple to do :-
http://www.carcomputer.co.uk
^^ They have everything you need to do this although you may want to source your kit yourself to save money. Technically all you need is:-
Chassis - Something small and compact but big enough to contain the following:
Motherboard - Mini ITX would be ideal scale
Processor - Just something that will fit the Mini ITX motherboard and runs cool
Memory - 1gb Memory should be ample
HDD - a Slim 3.5" Hard disk or even a Laptop Hard disk would be ideal
VDU - You need a Touch screen display preferably with USB output to go to PC.
PSU - You should be able to get an external PSU that will run off 12V DC no problem if thats not an option get a 12dc to 230-250AC Converter
OS - For this I would recommend 'Windows XP Ultimate - Lite' Which can be found on torrent. This is a slim cut down no 'Junk' Version of windows XP and boots up in litterally 5-10seconds on a moderate spec machine.
Also see this thread :-
https://www.fiatforum.com/i-c-e/117202-car-puter-car-pc.html
And pay particullar attention to the link I posted think it may help and give you an Idea how to procede

. Personally I could knock a professional looking and running system in about a month but just don't have the cash or time to do it.