Question: I have a Windows 7 Ultimate machine where the wireless adapter all of a sudden started having trouble connecting to wireless networks. Whenever I go to a new place and try to connect to a wireless network, it says that the DNS server is not responding, and tells me to go unplug the router and try again. After several locations in a row telling me this, I began to realize something was wrong with my adapter, not the routers. 

I am no longer asked to identify the security level for any new networks (Work, Home, or Public) like I used to be (it defaults to Public now - with the park bench icon). Often, resetting the router doesn't even work. Running the Windows 7 troubleshooter doesn't give me anything better than the advice to reset the router. However, the adapter will still connect to the wireless network at my main office without any problems. 

Does anyone know why a wireless network adapter can get so finicky so suddenly? 

Thanks!

Answer: 
It really depends on the case you have. 

I would personally get one of solid state picoPSU things and power the PC through a battery. Charge up a capacitor while the car is running, then when the car shuts off let it slowly drain through a resistor to ground and when the capacitor is empty get a mosfet to poke the "power button" pins for it to go into automatic powerdown

Note: I would try to avoid going DC(car) to AC(inverter/UPS) to DC(pc) as you'll loose alot of power that way and inverters always pull power even if they are not powering anything
I think, given the sheer form factor of a M-ITX case you would be better off getting some soft of UPS that is plugged into the car battery and working out how to do a graceful shutdown if the ignition has been cut for more than x minutes. You could try and bodge a load of batteries into a hard disk space but that probably wouldn't be a high enough current/voltage to power a PC.

Also consider a laptop. Inbuilt battery. See if you can find a model that can also take an external secondary battery.