So do you have a vailid copy of Windows? Have you installed Windows on more
than one computer? Have you recently re-installed Windows after making
hardware changes?
Actually a valid CD key will come to you on the Microsoft Site. In fact its already on your Microsoft CD. Just insert the CD and do an Advanced search for a .txt file(text document) looking in hidden files and folders.
The CD key is either in the Liscence.txt or Unattend.txt file.