Hello dreamer!
You are on the right track concerning drivers.
There are 2 options, first; leave AHCI set for Windows 7 and set it to IDE for the install of XP, then you would have to set AHCI/IDE in the BIOS every time you wanted to start one or the other, a major PITA in the long run, take my word for it.
Second, leave it set as AHCI and slip the XP SATA drivers into the XP installation media to get XP to install successfully, have a look at this tutorial at the link below and be sure to post back with any further questions you may have and to keep us informed.
SATA Drivers - Slipstream into Windows XP CD