Recover Lost CD Key

If you lose the CD case for the NT install, you also lose the CD Key unless
you’ve written it down somewhere. This is a problem when you need to reinstall.

• The CD Key is digits 6 through 15 of HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows
  NT\CurrentVersion\ProductId .
  
• Using Windows NT Diagnostics, the CD Key is digits 6 through 15 on the
  fourth line of information on the Version tab.