As amgat posted you will need the downgrade rights. To take advantage of your Windows 8 Pro downgrade rights, you need installation media (such as DVD or USB key) for Windows 7 Professional or Windows Vista Business. You also need a valid product key for the older OS.
If you have installation discs for Windows 7 Professional or Vista Business, you are almost ready to downgrade. As with any Windows installation, make sure any personal files already on your PC are backed up.
Once you're set to downgrade, insert your Windows 7/Vista installation disc or USB drive and install the system as you normally would. When prompted for a product key, use the Windows 7/Vista key that came with the older OS' installation discs and not the newer Windows 8 key. You will need your Windows 8 key after Windows 7 is installed.
Regular online activation for Windows 7/Vista will fail because you used a product key that is already in use or was in use at a prior date on a different machine. So now you have to activate your Windows 7 downgrade installation by telephone.
Your computer should automatically alert you that online activation failed. If it doesn't you can manually activate your machine in Windows 7 by clicking Start > Control Panel. In the search box of the Control Panel window type “activate” and hit enter. Click the link to “Activate Windows.” When you try to activate online, the process will fail and you should see a window with a bunch of empty text entry boxes as well as a phone number for the Microsoft helpline.
During the helpline call you may have to speak with a customer representative. Just explain the situation and have your Windows 8 Pro product key available to prove your PC has downgrade rights.