How to install two operating system in one computer?

i have tried to install xp and windows 7 on both the computer but when i install windows 7 first and than xp ,only xp starts and when i installed xp first then windows 7,only windows 7 starts there is no option of choosing os in boot. My friend have installed two os in one computer but i am not able to do.My sister likes xp and i like windows 7 to use is there any way that i can install both and get option of choosing os in booting time...please hlp me