Sometimes OEMs (like Dell) will use the same batch of product keys for a series of computers. I have more than one Dell desktop with the same product key on the sticker on top of the desktop case. The way to get the real product key registered to your computer is to download and run "Belarc Advisor" and then note down the product key. At this point you can go to Control Panel ---> System, and select "Change product key" under 'Windows Activation'. Make sure you enter the characters correctly; some are hard to read.
if you have a dell, and if you are using dell media, I.E. Windows 7 install disc that is Dell branded, it will install with the key embedded in the bios and reactivation should be unnecessary.