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.
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.
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. To change the default startup option, see.