You need to set the BIOS to AHCI before installing Windows 7. However if you do that your XP installation will BSOD the next time you try to access it. The only benefit I'm finding with AHCI is that it seems to make the TRIM command work more efficiently (assuming the SSD supports TRIM).
You can reset your XP installation to AHCI retrospectively by following the steps here
If you want to switch a Vista or Windows 7 installation from AHCI to IDE or vice versa then Microsoft have released this excellent tool