Win XP Service pack installer

[urgas]

Hey,

I have WinXP and I installed it from my back-up (yes of course I did). However, I am recieving problems when I install the service pack. Quite simply, I get complains that my Serial is not right and that I need to contact my retailer with the problem.
I'm not too found of that solution really.
Did anyone else have the same problem? How can I get around it. I do think installing the service pack would be a good idea.

Regards.