Have you tried to activate over the internet? Some times it will work aslong as you do not have a Dell/Gateway/HP/Etc. version of windows.
If you got a OEM form a small computer store or a place like Newegg you can reactivate over the phone.
If you have a Dell/HP/Gateway/Etc. version of Windows you will not be able to activate it thru the internet or phone. You will have to buy a new version.