First and most important, you'll need the new MB's driver cd to load the the chipset drivers on the old HD(with old MB drivers) and also your XP install cd(in case XP needs a driver for the AMD CPU). Then without internet connection you have to call Microsoft to reactivate. I just put all my old hardware on a new AMD based MB 3-4 months ago and the process worked Ok for me. Here is activation procedure by phone from Microsoft
http://support.microsoft.com/kb/307890