Well it depends on the fact that which storage medium are you willing to boot from. If you are not installing Windows 7 and it is already installed on your hard drive then set the boot sequence to:
CD/DVD Rom
USB
Hardrive
...
Make sure that you don't have any bootable disk or USB connected while starting your system, so it will automatically pick the hardrive. Whenever you will be willing to boot from DVD or USB then you just have to connect it/insert it and start your system!