the cheapest way to start out is to gut and old desktop PC, use the mobo and anything else that's useful, and put that in your car some how. With a full size PC, you'll have to either mount most of the electronics in the trunk or under a seat, and then run extension wires to your head unit, where you will most likely replace your stock radio with a touch screen.
Depending on the setup of your dash, you might want to look at mini-ITX mobo's. They're 17cm square, and with a double din radio slot you can normally do a dash install. I have a mustang 05+ and there is tons of room in the dash to put a pc, hence I went with the mini-itx. they can be had for cheap ( i think I payed 150 for mine) when you compare them to performance atx boards and I think they're well worth it.
Hope that gets you started.