Are you sure that you can't find any 12V regulators?
Do you intend on using this computer with the car turned off at all?
If not, then it's really easy. You can just grab a few 12V linear regulators and hook them up (you can get 1A regulators pretty easily, you'll only need 1 or 2 of them).
If you plan on using the computer with the car turned off then it is not a trivial task. I just posted a lot about this in another thread just today.
If anyone knows where to find cheap DC-DC AT PSUs, speak now
Look for my last post, it'll explain what needs to be done.