This is my current CarPC build. The system is powered by a low power Atom 330 Intel board. Originally running the Centrafuse 3 front end under Windows 7, it has since been upgraded to Centrafuse 4 and Windows 8. With a 30 GB SSD and reasonably fast post times, it boots up and resumes music/radio playback within 30-45 seconds of ignition. It's been running pretty faithfully since early 2011, though I pulled the system in 2012 and tried upgrading to an Atom 525MW motherboard. Although the system performed a bit smoother, the post times were far worse, so I went back to this original setup.
As an in-car infotainment system it features: AM/FM/MP3 Player, Navigation, OBDII diagnostics, Bluetooth, voice commands, etc. All the typical stuff you expect from a nice OEM or aftermarket head unit.
As a PC it features any Windows app you'd like to embed or just bring up... And of course, fun debugging sessions to keep it operating at happy levels when you foolishly think "Yeah, that [new driver][Windows update][cool app] seems safe to install!" Good times.