For everybody trying to install XP on a VirtualBox SATA harddrive, here's the floppy image containing the drivers :
Unzip and mount the image in the virtual floppy drive. Press F6 during install and select Intel ICH8M-E/M SATA
Tested on VirtualBox 3.2.10.
Or you can get the same drivers sans the ready-made image from Intel's site.