If you got the key from an OEM when buying a new computer, you may or may not have downgrade rights, it's up to the computer manufacturer: http://www.microsoft.com/oem/en/licensing/sblicensing/pages/downgrade_rights.aspx
I remember a few Lenovos my my company bought a couple of years ago came with a Windows 8 key pre-programed on the BIOS and an aditional sticker with a Windows 7 key.
If you got the key from an OEM when buying a new computer, you may or may not have downgrade rights, it's up to the computer manufacturer: http://www.microsoft.com/oem/en/licensing/sblicensing/pages/downgrade_rights.aspx
I remember a few Lenovos my my company bought a couple of years ago came with a Windows 8 key pre-programed on the BIOS and an aditional sticker with a Windows 7 key.