as a matter of fact "carputer" just describes a PC (personal computer) controlling various things (generally multimedia of course) in your car. Most setups faeture a pc small enough to fit in the 2din area hidden by a 7'' monitor (generally touch screen).
it's one of those uber geek stuff which has some great advantages and some very huge shortcomings. First you must be exceptionally skilled (and exceptionally geek) in order to properly setup a carputer (you must select proper computer components and you must find some obscure hardware accessories in order to make, say, steering wheel controls work, etc); you then have the freedom to fiddle with the actual interface. there are many frontend interfaces for carputers, because plain win7 is just way too awkward to use with touch input alone. and you don't want to control your head unit with keyboard and mouse!
and of course you have to wire all the stuff and connect everything properly to the speakers and the other components around the car.
your budget may vary alot depending on what kind of hardware you decide to use (which in turn depends on what you actually want to do with your unit).
All in all I would strongly recommend against going this route unless you are very skilled in computers and general DIY. the carputer ecosystem never expanded properly because big software developers / hardware vendors never joined the party.
I'd rather wait for an Android powered head unit.