Hard drive swap should not matter. The "key" is tied to the MB, but it's not a brand specific type deal. Find yourself an OEM XP install disc (same version..i.e. home or pro, whichever...SP doesn't matter) and you should be good to go. You can torrent a non-cracked one, but that's where we stop on this forum since it's "technically" illegal (even though it'd be the same thing microsoft or hp would send you). You'd need to use google for any instructions on that.