Are you using XP as OS. If so I am afraid changing the M/B means that the XP OS would not recognise it as it is deemed a major change. This is one of XP's features to prevent you from running one licence on more than one machine. Your only option, if that is the case, is to save as much data as you can using the old M/B and then install XP from scratch on the new system and transfer you data over. You'll have to activate again I am afraid.