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There is usually a sticker someplace on the laptop that gives the product key.
What manufacturer is it? For Windows 8 they sometimes have one product key for each manufacturer
In that case you can call dell. 1(###) ###-####/p>
they can send you a usb flash drive with an operating system on it if they cant give you a key number
another option is to load windows 10 on it which wont require a key
use the media creation tool on this page
do NOT choose the option to "upgrade this pc"
below that on the page is a button to download the tool
that will create a usb drive ( using a different computer)
put that in the problem computer and boot off it to install the windows 10
it should find the old key number but even if it does not you can just skip the part where it asks for a key or leave it blank and it will still install.