NTLDR is the boot loader for Windows OS. NTLDR missing means the boot loader files are missing or have been overwritten by some other OS installation. Here are a few ways to solve this problem:
->Put in your windows CD during boot. Press R to enter Recovery mode. Log into your existing windows partition, it should be listed as option 1. When asked for a admin password simply press enter, unless you have set a specific password which is unlikely. Now run the following two commands : fixboot, fixmbr. Then type exit to reboot and remove the windows cd from your cdrom drive.
->Another solution is once inside the recovery console, type the following commands:
copy e:\i386\ntldr c:\
copy e:\i386\ntdetect.com c:\
where "e" is the cdrom drive letter and "c" is your windows xp partition drive letter. You can type map command in the recovery console to see the drive letters on your machine.