Only major hardware changes,like changing everything at once, or the motherboard should prompt you. And you have to reinstall or buy a new copy if you change the board anyway. XP NEVER prompted for harddrives or addin cards by the way. That wouldn't make sense. Maybe the CPU
As long as it is on the same computer. Microsoft defines a computer essentially by it's motherboard. There is also a limit on how many times you can reactivate a copy of Windows online. At some point you will have to call them to activate (I believe the number is 5, but I'm not 100% sure).