I recently had a friend who wanted to find out his windows product key. his sticker had worn off his computer and he needed to reinstall Windows. There is a great tool for this called the Magical Jellybean Keyfinder. I have been using it for years and it has just been a great tool. It will not only find wour Operating System product key but it will also find the keys for all Windows based products such as Office, Visio and Project. Its a simple tool to get and use but it is a real blessing in many IT situations.
The web site for the tool is here.
Just click the download link and download the file. Unzip it and you will get a small .exe file which does not need to be installed. I keep this tool and many others like it on a thumb drive which I carry with me at all times.
Simply double click the file and you will get a windows with your keys.
If you want to save the results to a file simply click the file button and save as a .txt file.
As far as I know this works on all versions of windows.