Vista uses a brand new boot loader ...if you had XP and then installed Vista
it overwrote XP bootloader and replaced it with the Vista
version..........remove the HD and you have removed the boot manager..
I believeYou need to do a repair install of XP to correct the problem once
you have the XP drive in the master position...unless your running SATA
drives.
I am not sure if running the Recovery Console off the XP CD and running the
command "fixboot" would fix this.
peter