Boot Loader or NT Loader or NTLDR is a program necessary for XP, Win 2003 Server, Win 200, and Win NT 4.0 to boot up or make boot selections. These NT based operating systems requires two files for booting up, namely NTLDR and NTDETECT.COM. If or either NTLDR and NTDETECT.COM is or are missing or corrupted, the operating system reports after POST a text string indicating that your Operating System can't be loaded.
Corrupted or missing NTLDR or and NTDETECT.COM is caused by either hardware or software problem. IF hardware related, the power supply or the hard drive should the most probable cause of the problem. And in some instances, a virus attack could have done the fault.