raid : commonly used to have failsafe/backup hardrive. for eample raid1, you use two or more hard drives to store your data, all of the hard drives have the exact same data, so if one of those hard drives fail, you won't lose any data because the other one still works. so if you don't use RAID, you can turn it off
AHCI: never really looked into that one, but
http://en.wikipedia.org/wiki/Ahci .
"The Advanced Host Controller Interface (AHCI) is a hardware mechanism that allows software to communicate with Serial ATA (SATA) devices (such as host bus adapters) that are designed to offer features not offered by Parallel ATA (PATA) controllers, such as hot-plugging and native command queuing. "
Sounds like if enabled you have these fancy new features that XP will not probably even support, but when disabled, it's just an ordinary hard drive :[
My friend had this same problem and disabling ahci worked for him. after the xp install he just turned it back on.