Generic OEM? or did you purchase a Dell installation disk to reinstall the OS on an existing Dell computer that came with Windows 7? If generic OEM, then it should have come with a product key. If a Dell OEM disc, you should not need to enter a product key, but if you are prompted for one, then you will need to get the product key off the COA sticker on your computer.
I recently bought a OEM Windows 7 Ultimate SP1 64- Bit operating system, unfortunately I am unable to find any product key to conduct the installation on the DVD itself  or the packaging. I wonder where to obtain the key code or how to activate windows?  I am grateful for any advice!Sarah
OEM Windows 7 64 Bit- Product Activation