xp installed, vista installed, only vista opens.

hi frnds,
i want 2 have 2 os on my pc. so i installed xp sp2 1st nd den vista ultimate, but when computer starts , it doesnt show any options for selecting between os nd starts vista automatically. so i went to advanced properties in properties of my computer nd changes the values there, but now its showning options for vista & xp but selecting any of dem opens only xp, plz help.