Thanks for all the replies.
The guy who made the pc called us last night advising me to try a code from another pc he had.
Did it online and all activated fine.
Glad to hear you managed to get things sorted.
HP:
I can only go by my own experiences of using a XP OEM disc from one PC to another on at least three separate occasions without any problem.
Who knows, maybe on the one occasion I did have to ring MS I must have spoken to a very compassionate lady.