My friend had a Athlon sys build into his Audi TT and project was kinda easy when u look at it from outside tbh. So going back then also doing some research online reveals below.
The main problem is the power, signals and noise interference.
First Power, Depending on your setup you need to calculate how much power you need before u do anything. Car batterys runs on 12V and you need lot of batterys if you wanna run SLI setup
. If you need lets says around 600W power then you would need battery setup which can output 50AH * time you wanna use. Btw this is when your car is not running.
e.g Car is off.
600W / 12V= 50Amps
Typical car battery will have figures like
50AH@20 which means if you used your battery flat out in 20hours when car is not running at the end of that 20hours you would used around 50amps.
Car batterys output 12V at normal times. This can reduce to 9v when you start your car and change between 8v-13V depending on what you are running. So as you can see it changes too often so this means computer freezes/lock-ups/restarts etc. So basically you need to have another battery running for your computer so it doesn effect your car at all. This second battery also needs to charge up so i cant remember how the alternator setup was so you ll need to look into that.
Anyway so now you have your second battery and its charging fine, now you would need to use is an inverter which has plugs in them just like normal wall sockets. They come in lots of different sizes ( Wattage ) and draws its power from cigarette lighters to hooked direct to a battery depending on the size of the inverter.
Few inverters for you. ( These have british sockets in them some EU compatible tough )
2500Watts £400
http://upload.sysxtreme.com/0.X/0.999737/inv.2500.3000.12v.353.jpg
1200 Watts £100
http://upload.sysxtreme.com/0.X/0.999737/inv.1200.12.600.jpg
http://upload.sysxtreme.com/0.X/0.999737/inv.1200.12.add2.jpg
http://upload.sysxtreme.com/0.X/0.999737/inv.1200.12.add3.jpg
150 Watts £30
http://upload.sysxtreme.com/0.X/0.1138117/150.jpg
75 Watts £16
http://upload.sysxtreme.com/0.X/0.1138117/75.jpg
(Above is very handy to be honest we had a similar one to charge up or use mp3 players etc , now you could probably run your i-pod , PSP, mp3 players , digital cameras etc. )
Search on the net for inverters and you will see many of them and when you find one make sure it is a good one , similar to cheap PC Power supplies there are cheap inverters. Make sure your inverter battery setups output is more then what ever psu you gonna use for your pc.
Inverters Link:
http://www.exeltech.com/
If you dont want to deal with all these there are few companies who does vehicle pcs along with the power requirements.
Like
http://www.logicsupply.com/default.php/cPath/50
http://www.opussolutions.com/
http://www.karpc.com/
http://www.indashpc.org/new/
http://www.mp3car.com/vbulletin/index.php Excellent forum on car audio and carpc
Other problems are signal interference and noise caused by grouding and all the electrical cables in the car itself. So basically you need to separate power cables from audio and video cables. E.g if you have your pc in the trunk so you would run your power cables on left and audio video on the right or vice versa.
The rest is pretty much is easy same as normal home pcs. You can have £10000 worth pc or £300. Setup we used was we had the inverter in the boot with 2 batterys on each side of the car boot ( total 4 batterys ) cant remember how they hook up the batterys to alternators as i wasnt there perhaps speak to a mechanic
then we had the pc box hook in the middle of both front seats with custom holders so it doesnt shake about. You might wanna use something like suspension springs so it doesnt move the components inside the case when you go over the humps etc. Of course this depends on your setup. Then we had video / audio switcher and splitter ( booster ) basically you can combine many inputs outputs with this if you have multiple screens. All the video outputs run thru switch box as above. All audio re-routed to the amplifier , and speakers connected to the amplifier with crossovers and so on.... As you can see for your £180 pounds this kind of carpc looks too expensive but then again i am not pro in this and only seen 1 type of installation. Perhaps checking above forums might give you more information , perhaps a budget system or components.
Here are few setups you could use for your audio.
Bringing all outputs into Switch Box then to Amplifier
Head unit Output > Switch box
Computer Output > Switch box
Other outputs > Switch box then
Switch box Out > In Amplifier Out > Speakers
------------------------------------------------------------
Running your head unit into computer then to Amplifier
Head unit output > Computer Aux in
Computer out > Amplifier Aux In
Amplifier out > Speakers.
----------------------------------------------------------
No head unit just using your computer.
Touch Screen Monitor > PC
PC > Amplifier > Speakers.
Now FRAG i am sure you ll be insterested in this and for some reason i get the feeling this is gonna be your next project.
Hardware for Screens and usual setup:
1 x Main Screen in middle , used for general usage, online gaming, console gaming, gps, internet.
2 x Small screens at the back of the seat for whatever passengers wanna watch
1 x Car View Camera Monitor ( E.G Rear View, Front View or Both )
2 x Power Amps 1 for sorround sound 1 for Sub or bass box
7 x Normal Quality Speakers
2 x Bass Box or 1 Depends on power
1 x Equalisier
1 x Video Splitter
4 x Batterys
2 x Inverters 1 high output 1 low output.
PC Setup:
Usual gear + TV card , Radio Card + Bluetooth or wireless adapter , car antenna for tv and Radio with RGB connectors. IR Box and probably IR mouse , keyboard and controller.
2 x cameras 1 for back 1 for front
and of course your favoruite console with shit loads of cables , now you imagine what you could have in the car.
As for controlling cars computer or ecu you would need to find your cars OBD-II port and then use a OBD-II port to serial cable or OBD-II port to usb cable to connect your car to your pc. Then using the software you can view many things which the sensors in your car outputs. General software and normal standard ECU can only have usage and usual data , i am sure custom ECU's with custom software can make changes to every aspect of your engine and other stuff. E.G Fast and furious
Btw you can also reset service lights etc with some of these hint! hint! and get that mot or pass the inspection.
This is how OBD-II port looks like. This is on Audi A4
http://autocenter.weber.edu/OBD-CH/documents/newdocuments/audi/A4/2005_audi_a4.jpg
Check your vehicles OBD-II port from below address:
http://www.autocenter.weber.edu/OBD-CH/vehicleoems.asp
Software for reading OBD-II Data and few screenshots.
Homepage
http://www.obddiagnostics.com/
Screenshots :
http://www.obddiagnostics.com/ScrnShot/Winscreen.html
Other Sites:
Free Softwares:
http://www.easyobdii.com/_mgxroot/page_10767.html
http://ca.geocities.com/t_kolody/OBDSoftware.htm
http://www.qcontinuum.org/obdgauge/ < For PalmOS
http://pages.infinit.net/jsenk/obd_Win.htm
http://www.wgsoft.de/
http://www.fiero-gt.com/obd2.html
http://www.geocities.com/huffyfire/obd2/index.html
NON Free EEC Stuff:
http://eecanalyzer.webhop.net/
http://www.elmoscan.net/english/index.html
http://www.obd2spy.com/ also has free version and looks good.
http://www.palmerperformance.com/index.php
http://www.myscantool.com/details.html
http://www.vitalengineering.co.uk/software.htm
Post is growing bigger and bigger and it seems subject is getting lost so now i shut up
and use a laptop, double sided sticky tape / velcro to mount it on yer dashboard 
and one of those pencils things that Stephen hawkins uses to type with. 
(have they banned the use of pcs in cars?) whilst driving
