If I hold shift while starting up to boot into safe mode, it present a screen saying this in the background: When attempting to start up normally, it will get to the loading screen, show the Apple logo and the loading circle, then go black and not do anything more.
MacBook1,1 (Core Duo) 2.0 GHz upgraded to 2 GB RAM and 500 GB storage, running OSX 10.6.8 Snow Leopard