How to find out Windows product key after installation

This post is about how you can find out the Windows product key. Sometimes you might need your Windows key for upgrading your operating system.
What is a product key?
A product key is a 25 alphanumeric characters . It certifies that a copy of program is original.
How to find out product key?
You have to install a software called belarc advisor.  This software is of only 3 mb and can find out Windows 8.1 ,8 ,7, Vista,   xp ,2003,2000,98 product key.
Browsers supported by it are almost all.
You just have to install this software and this software will do every thing for you.
To download this software click here