I recently had a MOBO failure so put it in for a repair and upgrade from ME to XP. I have the COA product key and my repairer assures me that it is genuine, but when I try to install SP1 it tells me that the product key is invalid. When viewing the product key on the PC it is different from the one on the COA. Any advice please.
Just to double check. Is it a genuine XP CD with the holograms on it and the product key also a printed one on the rear of the jewel case?
If so, make sure you enter the key in lower/upper case as required and don't mistake an o for a 0 [Zero]! same applies to the letter L this can be a lower case like l or a number "One"  can be confusing, they look the same, and most of us have made this mistake before. j.
COA [certificate of authenticity] is different than the "Product key". The product key will not be available on your system display through normal means, and requires other software to access.
If you have XP installed on your PC, then a product key will have been used to install the software. This should be the same key as on the rear of the CD case.
Did you have to "Activate" the software yourself or was this done for you by the person up-grading your system?
If you have any doubts please give Microsoft a phone call on 0870 60 10 100 Explain the problem your having, they will tell you how to check both the CD and how to access the number on your PC through a sequence of key inputs.
Please don't worry about calling them, they are very helpful and it may just be a simple problem they can fix over the phone for you.
If the worst thing happens and they say you have an unauthorised copy, don't worry they will not shoot you, or ask awkward questions. They will just inform you of the facts. Good luck j.