You can't disable it, you won't have any video. Start in Safe Mode with Networking, you should be able to start it up that way.
Download the new drivers, but don't install them yet.
Uninstall the ATI drivers, don't reboot when it asks you to. Remove the hardware from device manager, reboot the laptop, and start in Safe Mode again. Install the drivers, reboot, see if laptop will start up normally now.