Hello Kumar.vivek,
Some manufacturers are putting the Certificate of Authenticity (CoA) with the Product Key inside the battery compartment on newer laptops, so have a look inside.
In my experience, if you are using the Dell Operating System reinstallion disc to do a clean installation of Windows, you are not asked to enter the product key, as the Dell OS disc employs the SLP method of activation.
Manufacturer preinstallations of Windows Vista and Windows 7 (like what came on your Dell laptop) can be reinstalled from the System Recovery Options menu accessible from the Safe Boot Options Menu. For more information on how to run the built-in recovery,
please visit the support page for your model of Dell here:
http://support.dell.com
