If you have installed Windows on your Mac operating system using Boot Camp, then you might want to switch to Windows at startup. Natively, Boot Camp will load Mac OS X at startup, but you can make the switch by simply pressing a button during startup.
Switching at Startup
Turn your Mac off.
Hold down the Alt (Option) key on your Mac keyboard.
Turn on your Mac computer. Select "Windows" to load Windows on boot-up.
Changing the Default Boot Option
Click the Apple logo when in Mac mode.
Click "System Preferences," and then click "Startup Disk."
Click either "Windows" or "Mac OS X" as your startup disk.