XP Pro Activation

Lightning toasted the motherboard in my wifes desktop, so we had a local computer shop fashion a new computer, WinXPPro, Sp2, 2gig mem. Everything was going along fine up until a few weeks ago. She started getting a warning on boot up that she only had X amount of days to activate. So we said activate and it always came back with "Windows is already activated" and boot up continued normally, except today. When her login window comes up it tells her she needs to activate and do you want to do it now? She clicks "Yes" and the screen goes blank for a few seconds and comes back with the "already activated" routine and takes her back to her login and starts the whole process all over again. Anyone have any magic dust to sprinkle? She can't even get into safe mode for the same reason.
Thank you for any help you can give

 

Thank you, Steve and Arie, for your help. As it turned out, I had to re-activate over the telephone.