Windows 7 is a commercial operating system developed by Microsoft. You need to buy a physical copy from authorized reseller or a digital copy from Microsoft online store in order to use it freely. When you reinstalled Windows 7, you can find the registration code from the physical CD, which prints out the code at the surface. If you bought the product from online store, then you should go back to the mailbox and search that email because the serial number was contained in the email after purchase.