I'm switching motherboards. What do I need to do to re validate the XP.

Try asking them here.http://social.answers.microsoft.com/Forums/en-US/xpinstall/threads?filter=alltypesRegards, Dave Patrick .... Microsoft Certified Professional -Microsoft MVP [Windows]

As Dave said, you should post Windows XP related topic in Windows XP Forum. General speaking, you just need to reinstall Windows XP. If it cannot be re-activated, contact Activation Center. Activation and registration of a Microsoft product http://support.microsoft.com/default.aspx?scid=kb;en-us;326851