Reinstalling Office XP Pro but msg says Product ID code Invalid

[Mitch]

I am attempting to reinstall Office XP Pro on the
computer that it was originally installed on. When I type
in the Product ID code when prompted, I get a message
that the product ID code is invalid. Now this I don't
understand. How can the PID be invalid when I type it in
exactly (multiple attempts) as it is printed on the
original product lable on the outside of the CD case? Is
there a way to force it to accept my code or bypass it?
Please help. Thanks,
Mitch