Exactly the drivers, Which you dont need if IDE is enabled in the BIOS. And you dont need them for Vista/Win7 or 8, if AHCI is enabled in the BIOS
It sounds like one hdd is fine (because AHCI is enabled, and windows installed the drivers.
Whereas XP (which is on the other hdd), since theyre on the same system, the OP would have to disable AHCI everytime he wants to boot into XP??
And thats what that reg file should do in XP. Allow AHCI to be enabled for XP. Thats if XP loads the SATA drivers after
But the SATA drivers wont be the reason (If dual booting isnt working).
The op (probably) disconnected the XP or WIn8 hdd, when they installed one of the OS's. If both dont appear in the boot menu