I have just purchased a used pc that was supplied with xp installed. It also has a windows installation cd labeled " Windows Genuine Advantage Kit for XP"
I started a fresh installation of xp but got to the screen where the product key was and found it was missing. I have the COA certificate with a 14 digit cetificate number but no key.
I now cannot get out of the XP setup screen as this keeps coming back when I restart the pc and I need the key to go further. I believe you can get into the registry to find the key but I cannot do this as mentioned above.
Any ideas gratefully received. Thanks

You could have obtained the existing key with various utilities before you started the install, but that's moot now. Without the product key, you're out of luck. The registry will have been "cleaned" out now.

These forums won't assist you in obtaining keys illegally. Your best bet is to either contact Microsoft to see if they will assist you, or purchase a new XP CD and start over.