Short version: Microsoft ties a HWID to your Windows 7 or 8.1 key and all of these use a single Windows 10 key which tells Windows 10 to activate via HWID.
Long version: All Windows 7 and 8.1 keys upgraded to Windows 10 are the same. That key tells Windows to go to Microsoft to check the Hardware ID (HWID). In short, your key is irrelevant. If you are installing Windows and it asks for a key, you should be able to skip it putting Windows into trial mode until you activate. When you activate, it should look up the HWID and use it to confirm your system.
But there's a catch: Microsoft obviously doesn't have an HWID without it being given to them and the only way to generate a HWID is to upgrade to Windows 10 from a functional Windows 7/8.1 install. Clean stall is impossible using a Windows 7/8.1 key until after Microsoft has the HWID.
If it refuses to activate and you tried a clean install off the bat. You'll have to reinstall Windows 7/8.1 then run the update to Windows 10. Verify it activated by right-clicking "This PC" and selecting Properties. If it did and you really want to install clean at this point, you should be able to and it will successfully activate.
Note: I tried installing Windows 10 from DVD and it failed repeatedly. Only installing from a USB stick worked for me (in terms of removable media generated from ISO anyway).