You would most likely have to re-install Windows on a new build. Did his old system come with the XP installation disc?
The only time I have not had to re-install was when changing out the motherboard for a similar make, usually one by the same manufacturer. If you get a same make mobo you might be ok, but if it is a whole new system then I wouldnt hold my breath.
I may be wrong though however, if someone knows of a way, but in my understanding you usually cant.