Total cost was around $1000..
The M2 is a smart PSU. It needs a switched (ignition) 12v, constant 12v (straight from battery) and a ground. When you switch the ignition off (or use an on/off switch to cut the power to the switched line), the PSU continues to power the car pc and sends a signal to the power button header on the motherboard. There are jumpers on the PSU where you can set a waiting time before the signal is sent from 4", 30", 5' and never. I have it set after 30".
The rest is windows XP setup, in power schemes mod, there are options what to do when the On/Off button is pressed, just like a normal pc. I have set it to hibernate. Therefore, when the PSU sends a pulse at the header on the motherboard, WindowsXP initialize the hibernation procedure.
You can have it to go to standby mode or do nothing as well, but the PSU will monitor the car's battery voltage level, and when it drops below a certain threshold, it will force the carputer to turn off, so that you can start the car later.