xp product key

why when my hard disk died i got another one(used-80-gig) installed it, then took my xp pro software disk, installed xp, looked fine, about 30 days later, i get threaten that if i dont regester it will, lock up. i get the product key of the cd, they (mircosoft say it is no good, then say it is, but contact the company who made my computer, now they want money to see if they can help me. is this normal, once you install a different hard disk, the product key doesnt work and neither mircosoft or hp will help