If you could post your motherboard specs we can supply you with the host serial ata controller in order for setup to interface with the disk.Once you have that downloaded and added you can get a copy of Link Removed
which will give you the options of choice to envoke bootmgr (Vista's boot manager) for winload.exe (Vista boot loader) or NTLDR Legacy bootloader which will entitle you to select at boot up which operating system you need to boot into. Once setup just always make a new Vista restore point after booting up to XP as the procedure erases all Vista restore points.
Ultimate User