Not entirely sure that I understand. When you bought the laptop from Dell, did it have Windows 7 or some other operating system on it? If it had Windows 7, Dell should have provided a way to re-initialize it, or will send you a disk if you call.
If it had some other OS on it and you installed Win7, the Win7 product key needed to activate Windows on the machine should have come with your media.
Or are you asking for something other than the Windows activation key? I can't tell if your problem is activation or just that you want to display some maintenance information from Dell.
i have installed window7 and window tag on back of my laptop and now product key is not visible hw can i get it i have dell inspiron and having product code 781L5BS
Look on the bottom of your laptop. All OEMs are required by law to provide you with the Windows Certificate Authenticity which has your product key on it. 99.99% of all laptop OEMs put the certificate on the bottom of the laptop.
Edit: I just checked my wife's Ispiron laptop and the certificate is located under the battery.