From Laidback website:
"No need for a shutdown controller if you use WinXP! Setup a generic UPS with a 'Negative Signal Polarity' in WinXP>Power Options, Then connect 5v from the PSU through a relay to pin 8 on the com port. Connect your 12v ACC line to activate the relay when your key is in the ingition. When you take your key out of the ignition, the ACC line drops to 0v which causes WinXP to close down. Simple!
For a quicker boot and shutdown enable Hibernation or STR/Standby."