Hello Buttseire,
The Product Key of the operating system can never be found on any website. The product key for the Windows7 version which was shipped with the computer would be on the COA Label (Certificate of Authenticity) stuck on the system tower.
Please confirm if you are trying to install the retail version of windows 7 operating system or from a Dell OEM disk?
If we try installing the operating system with Dell OEM version, the computer would not ask for a key. For the operating system with the retail version, the key would be on the disk.
Please write back for further questions.