Incorrect Product Key

I am trying to help my cousin who has an illegitimate WinXP Home SP2 installation. I purchased an OEM WinXP Home SP2 CD, and was originally going to do a repair install to preserve settings and apps. The repair option when booting from the CD does not appear (despite a few recommended changes /registry tweaks).

So I subsequently discovered I could change the Product Key by rendering the current key invalid through another registry change and reactivating Windows. Seemed simple enough, but when I launched the Product Activation Wizard and selected to Change Product Key, I get "Incorrect Product Key". I've tried it a couple of times and I'm quite sure I entered the key on the label with the new disk.

Is this an issue with the OEM CD vendor, or have I done something to cause this?

Thanks
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