HKEY_LOCAL_MACHINE\SOFTWARE\MICROSOFT\WINDOWSNT\CURRENTVERSION
Click on the CurrentVersion key in the left pane of regedit, and in the right pane scroll until you find the value of ProductKey. This is your CD Key (not to be confused with ProductId which is the number Microsoft assigned to you when you registered Windows. This is for Retail Versions). If this is an OEM version, the key will have OEM in it and MAY be called ProductId rather than ProductKey. Write down both values if you are unsure!
One more thing I should mention. If your version of Windows is an "upgrade" version, make sure you have your previous installation disks because setup will ask for them to verify eligibility for upgrade. Since you are about to do a clean install, there will not be a previous operating system installed. This is not a problem as long as you have your previous installation disks. Setup will prompt you accordingly.
Note, in W2K and after the product key/CD key entry is encrypted, use this or the newest version of keyfinder program to get at it instead of the above method.
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