service pack error

i am a student at Purdue university and it was through
them that i purchased windows XP....when i tried to
recently download the service pack 1 it came back saying
that my product number(?) was invalid....do i need to
reinstall XP...or does someone else have my number or is
it because i bought it through the university deal or am i
just doing something wrong

any help that anyone can provide would be greatly
apprecited

thanks