The easiest solution is usually to wipe the drive clean with DBAN and then install XP afterwards. Of course the solution depends on the exact problem you're having, so if that's not the working solution be sure to explain why you "cant install XP."
I baought acer5720 laptop.it has vindows vista.
but now i want to install windows xp. i cant install xp.
will anybody tell me what to do to install windows xp..