So while trying to dual boot, I mistakenly removed all other partitions and ended up overwriting everything with a linux. Finally eneded up also adding a temporary Windows 7. This laptop came with a pre-installed Windows 8, whose key I never noted. It is Dell Inspiron 15-3521. Is it still possible to get back that original Windows 8? And how?
The key is programmed into the BIOS. You can get it by querying the BIOS from Linux or using a utility like RWEverything in Windows.
* You should stay away from those linuxes ...
See here to download a Windows 8.1 .iso and prepare installation media, focusing on the Dell OEM license and the UEFI BIOS with SecureBoot:
A hint for the future, if testing Linux use a Virtual machine: