so you installed a dual boot system and everything seemed to work fine but upon reboot, the apple logo and spinning wheel is replaced with a dark grey screen that says please reboot.

99% of the time this is caused by a hardware incompatibility.

most times than not, it is the video card.
my experience has shown the ati 9600 to work every time and can be found all over the net.

also check the osx86 wiki (search google) for their compatibility list.

i have seen the eMachines w3650 work right out of the box.