What do you mean by "reactivate"? If you have previously
activated your windows and it says the same message again
there is something wrong, try reactivating to see if the
pop up stops.
Otherwise ignore it and see what will happen after the
three days pass.
Other software, besides Windows XP, has activation requirements. Is it
possible that you have a version of Office installed that requires
activation? This is a separate process from the Windows activation.
