How to change boot loader

Hi , I have a dell system pre installed with windows 7. today i installed windows xp on a different partition but once the system re started the system direactly logs into win xp . i think the boot loader has been changed to win xp. i am unable to boot into windows 7 nor get any option . can anyone please help me out on how to change my system back to default and change back to windows 7 loader .Thank you