If you have a Mac with BootCamp set up, you're able to choose between starting it with the Mac OS or with Windows when it boots. The instructions below detail the process.
1. With the Mac powered off, press the power button, and as soon as you hear the startup chime, press the 'option' key (also known as the 'alt') key, just to the left of the 'command' key. Hold it down until you see a progress bar in the middle of the screen.
2. When the startup options are displayed, use the arrow keys or the mouse to select the partition or disk you wish to start from. On a Mac with BootCamp, you should see a volume labeled 'Windows' or 'BootCamp' in addition to your main Mac OS startup volume (often simply 'Macintosh HD'). When the volume you want is selected, press 'enter' or click on it.
3. Remember, this only selects the startup volume for this boot up. If you restart the computer, and don't hold down the 'option' key, the default startup option will be chosen.