you need to install the AHCI drivers for your Sata controller during Windows XP setup. An install disc with SP3 and these drivers slipstreamed is the most convenient method. Or provide the drivers on a floppy disc (an USB FDD drive should do the trick, if the CD has SP3) which is the default method.
Apart from that:
Usually, the older version of Windows should be installed at first, then the newer one. Somewhat difficult with a computer that has a newer flavour of Windows preinstalled and comes without an install disc.
I think that last part is wrong my understanding Win7 acts like Vista and that Win7 should be first installed and your Bios changed to AHCI setting first and install Win7 then after that you would install XP SP3 last since both have different formats and boot settings. This should be the way to create a dual boot setup.