Basic Causes for NLTDR.com errors are:-
1. Computer is booting from a non-bootable source.
2. Computer hard disk drive is not properly setup in BIOS.
3. Corrupt NTLDR and/or NTDETECT.COM file.
4. Misconfiguration with the boot.ini file.
5. Attempting to upgrade from a Windows 95, 98, or ME computer that is using FAT32.
6. New hard disk drive being added.
7. Corrupt boot sector / master boot record.
8. Seriously corrupted version of W indows 2000 or Windows XP.
9. Loose or Faulty IDE/EIDE hard disk drive cable.
In your case basically you installed Ubuntu in your external hdd but due to misconfiguration during installation ubuntu wrote the grub bootloader to your system drive ( or MBR) . so the boot information in stored in your external drive and when computer boots up , bios searches for active primary partition with MBR and since you have installed MBR in ,external hardrive it cannot find the boot information from your internal drive as it has been deleted by ubuntu. Now you need to restore your MBR via following methods
1. Insert the Windows XP bootable CD into the computer.
2. When prompted to press any key to boot from the CD, press any key.
3. Once in the Windows XP setup menu press the "R" key to repair Windows.
4. Log into your Windows installation by pressing the "1" key and pressing enter.
5. You will then be prompted for your administrator password, enter that password.
6. Copy the below two files to the root directory of the primary hard disk. In the below example we are copying these files from the CD-ROM drive letter, which in this case is "e." This letter may be different on your computer.
copy e:\i386\ntldr c:\
copy e:\i386\ntdetect.com c:\
7. Once both of these files have been successfully copied, remove the CD from the computer and reboot.
Hopefully this works for you, !!