What happens if a user attempts to boot the computerand the message NTLDR is missing is observed on the screen?

NTLDR is the "Nt Loader". Since Windows XP is built on the Windows NT kernel, the NT Loader is necessary for Windows to boot.

The easiest way to repair this is to boot from your Windows XP CD, then take the SECOND "R" option. The first "R" will be "Attempt to repair windows using the recovery console" That is not the one that you want. Continue as if you were going to reinstall windows. Then, the XP setup will say "Searching your hard drive for previous Windows installations", and should find C:\Windows\. Select "R" to Repair at that point.

Note: You will need to do all of your windows updates again once you have done a repair of windows.