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This is required for computers that came with Windows 8 installed, but not for computers that were upgraded to 8 later. The UEFI is the interface that controls your computer, and is the modern version of a BIOS. Windows 7 doesn't support UEFI, so you'll need to set your UEFI settings to support the older BIOS method. The process is a different depending on your computer, but generally you can do this:  Press the ⊞ Win+C or swipe in from the right side of the screen to open the Charms bar. Select "Settings" and then "Change PC Settings." Click "Update and recovery," then select "Recovery." Click "Restart Now" underneath the "Advanced Startup" header. Select "Troubleshoot" when your computer reboots, then "Advanced options." Click "UEFI Firmware Settings" and then click "Restart." Find the "Legacy boot" or "BIOS mode" in the UEFI settings and enable it. Its location will vary by manufacturer. You'll see the traditional BIOS startup screen after rebooting, since you enabled it in the UEFI settings. The process for this will vary depending on your computer. Many computers will display a "BOOT" key during startup that will allow you to select a drive to boot from. If you don't see this, you'll need to press the BIOS or SETUP key. This will open your BIOS/UEFI menu. From here you can select the "Boot" menu and then choose your installation disc or drive. Press a key on your keyboard when prompted to begin the Windows 7 installation process. It will take a little while to load the initial setup. Choose your language and region settings in the first few Windows that appear. During the installation process, you'll be prompted to select a drive to install Windows 7 on. Choose the drive that currently has Windows 8 installed on it. Remember, this will delete everything that is currently stored on the drive. Towards the end of the installation process, you'll be prompted to enter your Windows 7 product key. Enter the key you obtained in the first section. If you have downgrade rights, this key does not have to be an unused key. After entering the key, Windows 7 will finish installing and you'll eventually be taken to the desktop. If you are using your downgrade rights, there's a good chance that you'll receive an "Activation Failed" message after Windows loads.

Summary:
Enable the legacy boot function in the UEFI settings menu. Insert the Windows 7 installation disc or USB drive and reboot. Boot from the disc or drive. Start the Windows 7 installation process. Proceed through the Windows 7 installation. Select the drive that has Windows 8 installed when prompted. Enter the Windows 7 Professional key you found when prompted. Complete the installation and load the desktop.