A Windows CD key is a 25-character code used to activate the Windows operating system on a computer. It is typically found on a sticker attached to the computer or on the packaging that came with the computer. The CD key is used to verify that the copy of Windows being installed is genuine and has not been used on another computer.
There are several reasons why you might need to check your Windows CD key. For example, you may need to do so if you are experiencing problems activating Windows, if you are reinstalling Windows, or if you are selling your computer.