So I have windows 7 installed on my macbook pro, and had it set so that it would run windows 7 by default. But after setting in the product key and restarting my laptop something went wrong: my macbook can't start windows now. The cd drive would make a sound as usual when it starts up, and the screen turns white. It then turns black for a second before it's suppose to show the windows loading screen, but then the cd drive makes its sound again and this cycle repeats. The only way I can operate the macbook is by holding option and using OSx. Can I get some help please? 