Hi Andrew,
did you purchase a machine with Windows 7 Home premium on it and you are just re-installing? if so then you should have an OEM COA sticker with the key. If you purchased a retail copy you should have received a COA with it with the key on. If neither of
the above apply and you have simply acquired a disc then you must purchase a license for Windows 7 Home Premium Retail edition (OEM is cheaper but can only legally be supplied with new hardware).