Software
There are a lot more options available to you with software than there are with hardware. After all, your Carputer is essentially a “normal” PC. Any operating system or application that will run on a home PC will run on your Carputer.
Your use of software will depend heavily on what you intend to use the computer for though. If you’re just going to be using it for media, and nothing else, then I would recommend a lightweight version of Linux such as Xubuntu, paired with some form of media playback suite such as XBMC:
This would give you a robust and quick loading PC which is great for playing back video and audio. It also has the advantage that both packages are completely free, you just have to download them from the Internet and install them on your new PC.
There’s nothing to stop you using Windows either. Windows 7 will run well on any PC made in the last 3 or 4 years, and the interface is a lot more visually appealing that earlier versions. Most touchscreens will also come with software for Windows 7 which allows you to control your PC without a mouse or keyboard. XBMC is also available for Windows as well (still free) so it’s a great way to get the media playing elements right as well as leaving your PC free to run other Windows applications.
If you’re going to use Windows, you could also make use of a great application called Centrafuse which gives a fantastic touch-based user interface, as well as supporting a whole bunch of features like GPS, hands-free calling and satellite radio (providing you have the hardware for it).