A free product key (free CD key) for Microsoft Windows would be really useful in a couple of situations.
A free Windows product key would be great if you've lost your product key but you're wanting to reinstall Windows. A free Windows CD key would really come in handy if you've borrowed a friend's copy of Windows 7 or Windows Vista and you'd like to install it on your own computer.
Several websites list product keys for operating systems like Windows 7, Windows Vista, Windows XP, Windows 2000 and more. The CD keys these websites provide are either created by a product key generator program or are duplicate product keys from valid copies of Windows.
No matter where they come from, using a product key other than the unique one that comes with an individual copy of Windows is illegal. Using a product key more than once is usually a violation of the software agreement you make with Microsoft when you use their operating systems.
Requiring a unique product key is Microsoft's way of ensuring that each copy of their operating system is used only once and that you, the consumer, is paying for the copy you are using.
If you have a legal copy of Windows but you've just lost your CD key, rest assured there are legal ways of finding your lost Windows product key.
Sorry to be the bearer of bad news, but installing a friend's legal copy of Windows on your computer is not legal. Your friend's product key is only valid for his or her copy of Windows. If you need Windows installed on your computer, you'll need to use a unique product key. The only legal way to obtain a unique product key is to purchase a new copy of Windows.