After re-format of XP, cant get windows to ac

Hello,
I have an odd problem. I have refomatted a computer .. all clean and correct .. and after I have activated windows and all that .. and I get to the log in screen .. I put in my password and then it tells me I need to activate windows, do I want to. I say yes and then it tell me windows is already activated .. and so on and so on .. I can never actualy log in to the user profile. Very Odd .. any thoughts??

Cheers and thanks ..Bruce