Its becomes important to know your serial key or product key for future reference. When you reinstall your purchased software you need your serial key make your software genuine or full version. You can use many method to retrieve your product key. But I will show you some of it.

1. Using the buid in "wmic" command:

     "wmic bios get serialnumber"’

   You can also get to know your PC model by wmic command

      "wmic csproduct get name"

2. By using third party software like  Magical jelly bean keyfinder

Install and scan your PC for software and its serial keys.

Note: You can find the program and other installation steps in this link

3. In other cases you can find serial key in windows Certificate Of Authenticity (COA) sticker on back of your laptop PC.