with GAG installed...the ntldr file will not matter. Gag will hide all boot files of one operating sytem from any other installed operating sytem. This keeps the boot loaders and Master Boot records clean from corruption by the other operating sytems files.

The XP ntldr file should install from the XP installation disc..If it doesn't then there is a work around to correct this problem. Mostly it is because of hardware issues.....ie, too many things moun ted to the motherboard. Try removing all but the basic pieces of hardware before installing XP then after the installation you can re-install it!