Something is going on!!... The "NTLDR is missing" message is spreading like wild fire... I had it and nothing but a repair installation helped. A bootdisk should help because NTLDR is a boot file and repacing it should help but doesn't!... At least it did not in my case, and I tried everything I know... I used a floppy bootdisk, also a boot repair with the Win XP CD, also copying new files with the Hirens Boot CD and again from a host system and nothing helped.. I had to repair the Installation to fix it... So apparently the message is missleading.. it makes you think the boot sector is corrupted while the problem is something else. It may be a virus, a malware, a bad Windows update, etc. but you need to do a repair installation to solve it.
If the CD Drive is not booting, try repairing Windows XP from a USB Pendrive.
http://bit.do/Repair-Install-Windows-XP-from-USB-drive