This is my experience when buy original windows in the market. Even you can buy an original windows in market actually you still can't guarantee that your window can work as original one, especially if you don't install your window program by yourself. It is possible that your windows product key already stolen, so you can run your windows update online smoothly. Even you tried to enter your window product key as sticked on the disk but you can run your update online.
Duplicate product key have been record on internet so that your windows will indicate as coppied or not original windows. The product key of your windows actually can be ask dirrectly in internet if any other same number on the internet then your product key actually have been use by another. Be careful before you buy original windows.
There is specific way to identify the original your windows online. How to Validate Windows 7 to check the authenticity of the Windows 7 Operating System you using? An easy way to see if the Windows 7 Original software is original must pass test in an oline services of the Genuine Microsoft Software.