When you install and activate a retail copy of Windows, the product key is stored in the registry. This can be helpful to be able to use a program like ShowKeyPlus to view your installed product key if lost.
However, having your installed product key in the registry could leave it vulnerable to being stolen from disclosure attacks by malicious software. Clearing the product key from the registry will prevent this, but you will need to make sure to keep a copy of your product key written down and kept in a secure location.
This tutorial will show you how to permanently clear the product key from the registry to prevent it from being stolen in Vista, Windows 7, Windows 8, and Windows 10.