It is safe to switch between AHCI (enhanced) and IDE (compatibility) mode, as it only changes the way the disk is accessed, but it doesn't change the data at all.
Windows 7 supports IDE mode, but it's not recommended because AHCI is faster. You can actually download the AHCI drivers for XP if you search for your motherboard (or computer model, especially if it's a laptop). Install those drivers, then reboot XP with AHCI enabled and it should work.