Windows 8 Pro comes with “downgrade rights.” If you’re not a fan of Windows 8 when you bought your new computer, you have the right to downgrade it to Windows 7 for free at no addtional charge but remember this only applys to Windows 8 Pro.
This can be a bit of a pain as Microsoft designed this procedure for business users. Home users or individual users will have to go through a number of hoops to downgrade their Windows 8 Pro systems and may have to contact windows admin support on the telephone.
Remember after downgrading to Windows 7, you can reinstall Windows 8 at any time.
Also You can only downgrade to Windows 7 Professional or Windows Vista Business, not cannot downgrade to Windows XP
Licensing Brief – Downgrade Rights