Yes - because the record *on the machine itself* of it having been done
has been lost (and no you cant back it up to reuse, because date of
install is part of it). But it will activate again on the net in a few
seconds, just like the first time.
Read up at www.aumha.org/win5/a/wpa.htm - and the hint in 'Formatting a
hard disk' about conserving one activation 'vote' when you do it