Thought I should pop back up again (although I really should be working!
)
Putting the whole PC in the Double DIN space is completely possible, I was evaluating a few ideas like this before I decided to put the PC in the boot space
I did this because I already had the PC, and at the time it seemed like the cheaper option, seeing as I wanted the PC to be as powerful as possible.
After having used mine, it's not completely necessary to have the power that I have in there and still be happy with the speed.
Using a Micro or Pico ITX motherboard it is possible to put the PC behind the screen, only limitations here is everything is very fiddly. However, this has some advantages as the harness is right there (the whole thing can be wired in like a standard head unit) and you save boot space (depends on the install).
Main disadvantage to this of course is it will cost more, like I said earlier, smaller = more expensive. Also for the BadBong's among us the smaller motherboards don't always have the space needed for a dedicated sound card ;) One possibility around this (that I never did) is the motherboard I used actually has Optical Audio out, so you could always pipe that straight to a decent DAC if you wanted the best sound quality.
For the tablet PC idea I've done a little more research.
There is dedicated software designed for android tablets for use in cars. I don't know if it's any good, I don't have an android tablet ;)
The tablet won't do anything special when you turn the car on, so you need to make sure you stop playing music on it before you hop out of the car or it'll go flat. Advantage to this is with a simple 12v to 5v power supply you can charge it simply, and just kill the power straight, rather than doing it safely like you do a PC.
It looks like some of you are really just looking for a cheap head unit, almost anything out there will be better than a cheap CarPC, if you want a good one, you do need to invest a fair bit of money, however it's not as expensive as a high end head unit.
Sorry if that post was a little bit of a ramble, my minds still half focused on work ;)
Sam