You may in fact have to call Microsoft about that,
Make sure you truly are using the right code with the CD you have. Sometimes you get a XP Key and you installed Windows XP SP2 and those two keys are not going to be the same. It’s not an easy one to figure out without you double checking the KEY vs. the INSTALL CD and calling Microsoft.
I would offer you my old CD and Key for free that I don’t use anymore, but I can’t seem to find it. Plus I am sure someone somewhere would have issues with that.
The other issue you will have is if you have a new set of keys, from the Rev C stuff from when Microsoft had to expand the XP numbers after Vista came out. Those keys will not work with the old CD’s. You have to have new Rev C media to work with the Rev C number. We have been experiencing this alot the last few days with our new builds.