You simply install windows XP. Then you repair Windows 7.
BTW, in order to install Windows XP/7, you need to boot from the CD, instead of booting from the hard drive. So, you need to go into BIOS and change the boot up priority. After you are done with XP, insert windows 7 CD, and choose repair installation. then change back the boot up order to the hard drive and you are done
the dual boot menu is created automatically, you dont have to do anything. Having said that, you may afterwards download an application for windows called EasyBCD, its free and allows you to change dual-boot settings.