have you tried uninstalling drivers from safe mode? try the uninstaller and manually removing them from device manager.
also in safe mode start msconfig, go into services and hide all microsoft services, disable everything that is not by microsoft. under start up disable every entry.
if it boots, then there is something wrong with either running service, ati driver or startup program
then slowly start enabling things back on, if it stops booting again, go back and see what caused it.
then switch to nvidia, less driver issues