Hi Gabriellw,
Windows 8 systems are pre-activated from factory. Once the PC is activated, it remains active unless there are significant hardware changes such as motherboard or hard drive replacement. If you re-install Windows on the computer, there is no need for the product key as it is encrypted on the motherboard from factory.
If you just need the Windows 8 recovery disc, you can create one for your computer from Dell Backup and Recovery. For more information, you may refer to link: http://bit.ly/12kIydx
Please reply in case you have any further questions.