ntldr is a system file necessary to boot the computer.
maybe you deleted or renamed it ?
if you can boot from a Win XP DVD, there is a copy of ntldr in the x386 folder on the DVD. Try copying to your hard disk which has win xp on it or the disk from where XP boot file are stored. (Usually C
Remove any external drives, usb sticks, CD, etc.., reboot and try again. Check the BIOS to make sure the hard drive is listed, if it is, set it as the first boot device in the BIOS boot order options. If it's NOT listed, reseat the drive connections.
If all that does not fix it, try resetting the BIOS (unplug system from power, remove the CMOS battery for about 15 seconds). If that does not fix it, you may have a bad drive, do a full disk scan with a drive check utility, there may be one in the BIOS if not the computer vendor may have one to download. If the disk checkes out OK, a repair setup of Windows may fix things.