Relatively easy. Use constant power for powering the PC's PSU, and use UPS software monitoring the ignition line to shut down windows. You'll need to build a small circuit to clean up the ignition line and protect the PC, and an appropiate serial cable between it and the PC. I'd recommend using apcupsd as the shutdown software - it will install and run happily on windows. Use 'simple' signalling. This monitors 2 lines on the serial port - one is 'on line power' and the other is 'battery ok'. If you derive both from the ignition line, as soon as you turn your car off, apcupsd will think that the PC is running on UPS batteries, and that those batteries are low, thus triggering shutdown.