Having personally replaced the hard disk drives in literally thousands of Dell's and others, I want to propose a different scenario about keys and activation. Dell and others create a master image of what they want on every one of that particular model of PC and then simply replicate it however many million times they want before the drives are installed during assembly. Embedded in that image is their, unlimited, manufacturing license key so it is already activated and, if you create the recovery media for reinstalling later, it has that same manufacturing license key embedded in the image. But; in order to satisfy Microsoft's licensing requirements, each manufacturer, Dell included, had to put a license sticker on each PC which has it's own, unique, key. This was true for Windows 2000, XP, Vista, and 7 after which Microsoft changed to the embedded SLIC model and there are no stickers on the PCs.
So, with Windows 7 or earlier, if you start with a generic operating system CD/DVD for installation, you need to use the license key on the sticker to activate or Windows will start screaming about not being genuine. With Windows 8 or 8.1, the key is pulled from the SLIC table in the BIOS and gets activated using that key. Windows 10 changed the model, yet again, in that it creates a hash of the specific hardware, checks it against a Microsoft licensing server, and will reactivate automatically the first time a fresh installation connects to the internet as long as the hash is reasonably close to the original hardware. (You can change the hard disk drive, upgrade the memory or CPU, or add a video card; but, don't do them all at once and you can't change the system board.)
Unfortunately; shortly after the release of Windows 10, Microsoft pulled the downloads for Windows 7 making it far more difficult to obtain a genuine Microsoft Windows 7 installation DVD. Luckily, HeiDoc.net has written a tool which will let you get those images named Windows ISO Downloader and it still lets you download an image of numerous versions of Windows and Office: https://www.heidoc.net/joomla/technology-science/microsoft/67-microsoft-windows-and-office-iso-download-tool
You need to be able to burn an ISO image to a DVD; but, since you have a valid license key, you can use a Windows 7 Professional DVD to install and then use the key to activate it.
I had asked about the PC's SERVICE TAG because there may be some hardware drivers which need to be downloaded and saved onto a working PC then copied to the fresh installation using a flash drive for it to work. Even though it is older than Windows 7 (2009), Realtek network chips have a nasty habit of not being functional until you install the drivers.
So, again, what is the model of the Dell or what is its service tag number?