Restore window vista

i was having vista on my laptop acer travel mate 2490. then i install window xp on same drive. now i cannot use vista.even on start when i press F8 to choice the operating system. there is only window xp. even in controll pannel add remove program i cant see the option to uninstall the win xp. i wants to restore factory setting. so that i can use vista. please tell me the simple sloution.