A new motherboard will certainly cause XP to want to re-activate, since that
almost always means a new CPU, new RAM, new NIC adapter, etc, etc. A retail
copy of XP can be reactivated on new hardware, an OEM copy can not.
You may need to call the Microsoft 800 number (which will appear on the
screen, only if the internet re-activation fails). But, that is no big
deal. The one time I needed to do this, the MS representative was actually
friendly. If it happen to you, just explain what you are doing.
See the following article for a case study in swapping motherboards:
http://www.extremetech.com/article2/0,1558,10333,00.asp
You should alos be prepared to do a "repair" installation of XP, if you
attempt to keep the current hard drive with the current XP installation and
programs. A new motherboard almost always requires the repair installation,
unless of course, you choose to do a clean installation, which will erase
all data on the target partition (usually C:\) before installing XP. As
always, have a backup of all personal files, just in case something goes
wrong during the upgrade.
Here are some links that might be useful:
http://support.microsoft.com/default.aspx?scid=kb;en-us;315341
http://www.webtree.ca/windowsxp/repair_xp.htm
http://www.extremetech.com/print_article/0,3998,a=23979,00.asp