win xp genuine, product key genuine… but why is it invalid?

i have a win xp prof sp2 laptop where i recently did a reformat… i followed all the steps in the book.. from the complete format of the c drive to installing… however now that its installed there is a popup saying that i have 30 days to activate my windows… trying to put in my product key.. it will not accept it… saying that it is invalid.. what do i have to do to get it validated? thanks before hand