From your post, it sounds like you haven't given much thought to how to power a PC in your car. This is the trickiest part to decide and implement on.
I suggest you head over to the FAQ Emporium and take a look at the "FAQ to the FAQ's". You can click on the link in my signature to go there directly. There are thread designed to get newbies up to speed on how to build a car PC and you'll also find several threads that explain the factors you need to take into consideration when powering a car PC.
There are also links to power calculators and deeper discussions on the differences between inverters and DC-DC power supplies, how to decide on the proper gauge of wire to use and advice on fusing and grounding properly.
When you're finished, you'll have an idea of what the right questions are to ask on the forum when assembling your PC. It's a great hobby and a lot of fun!