install xp right on top of 7 ur have a duel boot system
No you won't. XP's boot loader will overwrite 7's boot loader. XP's boot loader doesn't know anything about windows 7, so you won't be able to access win 7 anymore. You will end up with wasted disk space. To properly dual/mutli boot windows OS's, you always install the OS's in order from oldest to newest.