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
Step 1
Turn your Mac off.
Step 2
Hold down the Alt (Option) key on your Mac keyboard.
Step 3
Turn on your Mac computer. Select "Windows" to load Windows on boot-up.
Changing the Default Boot Option
Step 1
Click the Apple logo when in Mac mode.
Step 2
Click "System Preferences," and then click "Startup Disk."
Step 3
Click either "Windows" or "Mac OS X" as your startup disk.