I believe if you press F12 at startup, you will enter into a 'Boot Selection Menu', this allows you to choose if you want to boot from a cd, USB device or a hard drive. If you would like to permanently change the the boot order, if you enter the BIOS, under the Boot menu, you should see the options I mentioned above. You usually press '+' or '-' to move the selection up and down. The top or first (Depending on how your motherboard phrases it) will be the first location your computer looks for a boot device.