Licensing issue with Windows 7 reinstall

I have a Dell Latitude E6520 laptop that I decided to reinstall Windows 7 on. Before wiping the old installation, I grabbed info about the product keys from the registry using the NirSoft ProduKey tool (I thought this would be enough). One of the entries was "Windows 7 Professional" and it gave me a product ID and product key. I still have these.

So I wiped the old install and installed afresh from my Windows 7 retail ISO, selecting Windows 7 Professional to install. All went well until it came to product activation. When I try to activate Windows with that same product key, I get the error:

A problem occurred when Windows tried to activate. Error Code 0xC004E003.

I presume this is something to do with this laptop not just being licensed by the key but also by something in the BIOS? Or a subtly different OEM version of Windows 7 Professional as opposed to the retail version I installed? Am I screwed or is there any way I can license this?

Okay, so you bought the Dell with the Windows 7 OEM already pre-installed - the key they (Dell) use during manufacturing is likely NOT the same as the key behind the battery.

If you had installed from Windows 7 Dell OEM media, then it likely would NOT have prompted you for a key due to what it sees in the BIOS with that media to activate itself.

You reinstalled Windows 7 with retail ISO/media onto the Dell machine rather than Dell OEM media so it probably needed that key behind the battery to check in and confirm it's (the retail you used to install) authenticated.

I'm surprised the retail did NOT come with it's own key to use for this, and I'm also surprised the OEM key behind the battery allowed activation against the retail ISO media -- you may have just gotten lucky with the build and revision of the media but I've personally had trouble with this before so I always use manufacturer OEM media with Dells and HPs (at least with Windows 7).