1. Understand the causes of missing NTLDR errors. One of the most common reasons for an NTLDR-based startup error occurs when Windows XP attempts to start from a hard drive that isn't set up to launch Windows XP. For example, you may encounter an NTLDR error if you attempt to boot from an improperly formatted Windows XP startup disk. These errors may also be caused by corrupt, missing hard drive data and loose hardware cables.
2. Click the Start menu button in the lower left-hand corner of the Windows XP screen. Select 'Restart' to reboot the PC.

3. Check that the computer's optical media drives are empty. A major cause of NTLDR errors is when a non-startup disk is in the CD slot and Windows XP attempts to launch from it.
4. Insert the Windows XP installation disk. Restart the computer. Press any key on your keyboard during the startup process to launch the installation setup screen. Press the 'R' key on your keyboard when the Windows XP Setup screen appears. This launches the Windows XP Recovery Console program and a screen in which you can type command prompts.
5. Type 'copy d:\i386\ntldr c:\' (without quotation marks) in the command prompt window, then press the Enter key on your keyboard. Type 'copy d:\i386\ntdetect.com c:\' (without quotation marks), and press the Enter key again. The Windows XP Setup software will detect and replace any NTLDR software files that may be missing or corrupt.
6. Press the 'Y' key on your keyboard if the Windows XP Setup software asks for permission to rewrite and replace the files in Step 5.
7. Eject the Windows XP installation disk by pressing the Eject button on the front of your computer. Type 'exit' (without quotation marks) in the command prompt window to close the Setup software, then press the Enter key on your keyboard to restart the PC. Windows XP will now launch without missing NTLDR errors.