Help installed windows 7 twice

Ok i need some help here, i got windows 7 3 days ago and after installing it i used it for about and hour before a power surge blew my cpu and mother board, after i got a new one my hard drive wouldn't load past the windows 7 start screen it would just crash, after looking on the internet for what to do i was told to reinstall windows, and it worked but now it keep asking for a activation code which iv already used the first time so now it just keeps telling me that its not valid and i have 29 days to reg one, please can anybody help