Well, if you have one it will show here:
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion
click on the CurrentVersion key and in the RH Pane (listed alphabetically) look for ProductKey and alongside it will be the 5 groups of five letters and numbers.
If you bought your system from a local Ma and Pa computer store it probably came with a WiNME CD with the pretty holograms on it. This is the (original) O.E.M. installation disc. If you don't have that you may have something else - it would be interesting to know what.
The product key is needed when one does a format and install of WinME using the WinME CD.
Ben.
Don't like working in the Registry? Then this zip attachment is for you. It is a simple VBS (Visual Basic Script) tool which will instantly display the key. Courtesy Bill James MVP from his website here:
http://www.billsway.com/vbspage/