Well, I could offer some ideas. Scroll down to the HIDiouSDC project here and grab the source code: http://curioustech.home.insightbb.com/carpc.html
user.c has the SDC and other routines in it and hidsdc.h is the windows code. There's more to it for the OBD-II, relays, SWC, IR, radar detector and all that but you could rip all that out and just use the SDC part. In SDC_Sequencer() it checks ACC every 500ms, then if it's on, it checks again 500ms later before turning on the PC. There's also an instant on if a power button is pressed (or door unlock). The shutdown timer is set by software though.