Checking for a Certificate of Authenticity (CoA)
Before using any software methods to recover an XP/Vista/7 product key make sure to check the bottom and rear of your computer for a Windows Product Key Sticker like the one below. The string of numbers and letters on that sticker is your Windows Product Key and you should be able to reactivate using it.
Using Magical Jelly Bean Keyfinder
Open your Downloads folder and run the installer that was downloaded.
Launch Magical Jelly Bean Keyfinder by double-clicking the icon on your desktop.
Your product keys should be displayed in the new window that opened. Now record them somewhere.