Webstore, Windows 7/8 product invalid

Why does my Windows 7/8 product key say it is invalid?

Please verify that you are not confusing 0 (zero) for O (letter O) or 1 (number 1) for l (lower case L). Also, please remember that the version of Windows 7/8 we sell on the WebStore is an UPGRADE version and you should have a previous Windows operating system on the hard drive when you bootup from the Windows 7 disc during installation.