Sometimes – most notably when Mac OS X doesn’t start up normally – you need to boot from an optical disc (such as the installation disc that came with your Mac). To do so, insert the disc as soon as possible after pressing the power button and then hold down the C key. If Mac OS X is already running but you want to reboot from an optical disc (perhaps to repair your startup drive using the installation disc’s copy of Disk Utility), go to System Preferences and choose Startup Disk. Next, select the disc in the pane near the top, then click Restart.