1. First u know about your(HDD) system information which os is supporting your system.(some time HDD is not supporting xp) based on HDD type at a time any one windows OS only u install.
After u boot windows xp cd/dvd create partition like this c:,d:and e:
now u install your win-xp in C:/ drive.
after u boot vista dvd after loading file press Esc key now u see all drive , select d: or e: (don't disturb c:), like this u select d: now vista is installing your system, after you start your system now 2 menu is display like this
microsoft windows xp and microsoft vista ................. u login any one os
u want run both os at a time install microsoft virtual pc ..........
i know what your problem, you try to install xp in ahci but xp didn't recognize ahci driver because not loaded in installer. use nlite to add ahci driver to xp and burn into new cd. now you can install xp in ahci system.