Right the thing is i have just bought a computer for my car and will be installing it next week which is here.
I have everything i need e.g. screen (Kenwood KVT-M700) ect all i wanted to know was about the powering side of things. My friend has said to me i have two options.
1) Use a mains inverter. This creates a 240vac feed from the battery which you plug the mini-itx's external mains adaptor in to. This is a waste of energy though - power is lost in both conversions. Cheap inverters also work on a modified sine-wave so output a lot of noise on the supply which can cause buzzing through your speakers and interference on the screen.
2) Buy a voltage regulator. This will increase or decrease the voltage as necessary - it sits in between the car battery and the pc's 12v input. This is the best option. i have the regulator inside the pc case. The pc will run for 8 hours without the engine on!
Now i am not to clued up on this sort of things but the second option seems to be better to me. But have not got a clue what i voltage regulator is/does and where to get one from. If you can shed any light on this matter that would be great.