Well, it turns out that this XP was counterfeit after all. The Amazon-connected retailer claims:
Thank you for your email. I have have looking into this order extremely
thoroughly and have discovered that we had a number returns from the same customer last month, we now believe that he swapped our legitimate copies for counterfeit. I will also be contacting M/S first thing on Monday morning with a view to discussing what can be done, as I'm sure this would fall under fraud.
They claim they have done a refund to my CC.
I sent the package off to M$ 2 weeks ago but have heard nothing.
The crack I applied works OK for the O/S but Medial Player 11 won't install, saying it fails validation. So one day I am going to have to do it all over again.
Can one just install a new copy of XP over an old one, entering a new S/N, and preserving all application settings? Or should I reformat the whole partition?