If you have a Windows 7 COA then you won't have a Windows 8.1 UEFI BIOS embedded SLP key. You therefore will be unable to activate Windows 8.1 Pro. Windows 7 keys are not interchangeable with Windows 8.1 installation media but are with Windows 10 installation media. Since you have a Windows 7 key you can instead use it to directly clean install Windows 10 RS1. You should update your UEFI BIOS to the latest version and enable SecureBoot before performing the clean installation. The Inspiron 660 should run Windows 10 RS1 without a problem (despite the fact Dell don't support it). See here for details: [URL]  [URL] 
I bought an Inspiron 66 used on Craigslist with Windows 8.1 Pro with Media Center. Recently, I started to get warning screens that Windows validation was about was about to expire, which it subsequently did. After many hours of searching, I was able to determine that the Windows 8.1 Pro was installed with a generic volume license key from a KMS server (thanks, craigslist). Since the validation has expired, apparently, that key no longer exists. After many more hours of searching and being given the run around by Microsoft support technicians, I still do not have a valid key. The only clue I have is that there is a Dell Windows 7 Pro OA sticker attached to the computer case with a key.  It looks like the latest OS approved by Dell for the Inspiron 660 is Windows 8.1. Is there anything that can be done where I can keep the Windows 8.1 Pro (the Media Center is unimportant to me)? From what I have read, it appears that the key for the invalid Windows 8.1 Pro is embedded in the Bios and that any attempts to repair or reinstall the Windows 8.1 Pro on this computer will be tied to that expired generic key. Does anyone know if this is true and possible to overcome? I have searched endlessly but have not found any solutions for my situation so I thought I might post here and see if anyone has a solution but I'm afraid my situation is becoming time sensitive and I can no longer sift through thousands of suggestions that don't pertain to my situation. Windows has grown so tiresome. Thanks in advance and any information is appreciated. dan
Inspiron 660 Windows 8.1 Pro validation has expired