VistaBootPRO would be your answer, it provides a graphical front end to
BCDEdit Command line that makes it easy for you define startup settings and
edit boot entries on the new Windows Longhorn Server/Vista boot manager. http://www.pro-networks.org/vistabootpro/
There are two phases to this, I would suggest booting into XP, click Start >
right click My Computer > click Properties > Advanced (tab) > under "Start
up and Recovery" click Settings, change the "Time to display list of
operating systems:" to a shorttime limit, I would use 5 seconds > ok > ok