If you have installed Windows on your Mac operating system using Boot Camp, then you might want to switch to Windows at start-up. Natively, Boot Camp will load Mac OS X at start-up, but you can make the switch by simply pressing a button during start-up.
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.
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 start-up disk.