If you have an older machine that includes a floppy disk drive, then this could be caused by a non-bootable floppy installed in this drive. Remove all floppy disks before booting.
"Error: NTLDR or NTDETECT.COM Not Found when starting Windows"
< http://support.microsoft.com/kb/555304 >
Another possible cause is the following:
"Windows may not start and you may receive an "NTLDR is missing" error message if Windows is not up-to-date and there are too many files in the root folder"
<http://support.microsoft.com/kb/320397>
HTH,
JW