Vista 64bit previouly instaled. XP installation attempt. Reboot and get "Error Loading Operating System" error.
Check your motherboard manual for bootable SATA connectors. Temporarily remove all hard drives not associated with installation OR make sure the proper SATA is listed first in the HARD DISK boot order in the bios.
Sometimes boot order only shows one hard drive. You have to go to another item in the boot menu which shows you the boot order for all your sata hard drives.
This worked for me and I hope it works for you too. I had too many hard drives and they were probably hooked up to the wrong connectors.