Lost the product key that comes with your Windows installation disk? Not to worry — it is lurking deep inside Windows.

From your Start Menu, choose Run, then type REGEDIT in the box and
click OK.

For Windows 95, navigate to
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion and look for an entry in the right pane called “ProductId”

In Windows 98 and ME, navigate to
HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\CurrentVersion and look for an entry called “ProductKey”

In Windows 2000 and XP, there are in fact two entries, both called
“ProductKey”, one under
HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\CurrentVersion and another in
HKEY_LOCAL_MACHINE\Software\Microsoft\WinNT\CurrentVersion.