Buy the original CD, if have more problem , and contact the M$ like what HTRegz said .
It ever happened to me once when i put the wrong Licence Key, it wont upgrade and it said "Wrong License Number" so i reinstall the Win Xp and put the right number.
When u try to upgrade the XP they will look on yoru OS license number , if it is correct and hasnt used by any person, they will allow you to update yoru XP, if not they will ask you the rigth number.

Good Luck