Magical Jelly Bean Keyfinder 

Magical Jelly Bean Keyfinder is a free and handy application that retrieves product keys from the registry of any Windows installation.

During installation, it offers you to download and install a tool that is not required by Magical Jelly Bean Keyfinder to function. You can decline this though if you are careful and not click "Accept" by mistake.

The interface is simple and straightforward. On the left side it displays the Windows installations and on the right it lets you view information regarding each installed software, such as CD key, product ID, registered owner, registered organization and other.

You will also be able to load a registry hive of another Windows install. All you need to do is to select the Windows directory, press "Load Hive" and indicate it to the damaged Windows install. Once you load the hive, it will reveal the product keys from the registry of any Windows installation.

Not only it gives you access to the product keys for your Windows installations, but it also shows handy information about these installed applications.

Cutting a long story short, you should definitely try Magical Jelly Bean Keyfinder, free software that retrieves and displays the product keys of any Windows installations.