When you start up the machine, do you see something like "press del to enter setup"?
If so, press the relevant key and you will get into a series of menus. Within one of them you will see an option for boot order - set it so that it will boot from CD.
As for drivers, be sure that XP drivers are available before removing Vista; if they exist they should be on the Samsung site.
Out of interest, what is your difficulty with Vista?