You downloaded off of the net? It sounds like a "bogus copy"!
Each copy of Windows regardless of version has "it's own"! product key! To see this mess cleaned up you need to buy a full verion copy even OEM of Windows and registar that along with seeing it activated.
Other then that there's nothing else that can advised here. Once you have a legitimate full version disk you can get help here with any problems installing along with repartitioning and reformatting the drive to see a clean install. The invalid message is simply due to being a different copy of Windows where the first product key is now useless!