Missing or Corrupted System Files
For the Operating System to boot properly, system files are required to be safe.
In the case of Windows 95 / 98 / ME, these files are: msdos.sys, config.sys, autoexec.bat, system.ini, system.dat, user.dat, etc.
In the case of Windows NT / 2000 / XP, these files are: NTLDR, ntdetect.com, boot.ini. They are located in the root folder of the bootable volume, as-well-as Registry files (i.e., SAM, SECURITY, SYSTEM and SOFTWARE), etc.
If these files have been deleted, corrupted, or damaged by a virus, Windows will be unable to boot. An error message like "NTLDR is missing ..." will appear.
The next step in the recovery process is to check the existence and safety of system files (although it may not be possible to check them all, check for NTLDR, ntdetect.com, boot.ini which cause most problems).
To do it in Windows 95 / 98 / ME - boot into a Command Prompt Mode, or from a bootable floppy and check for system files in the command line or with the help of a third party recovery software.
To do it in Windows NT / 2000 / XP, use an Emergency Repair Process, Recovery Console or third party recovery software.
Emergency Repair Process
To proceed with an Emergency Repair Process, an Emergency Repair Disk (ERD) is required. It is recommended that this disk is created after installing and customizing Windows. To create it, use the "Backup" utility from System Tools. The ERD can be used to repair a damaged boot sector, a damaged MBR, or repair/replace a missing or damaged NT Loader (NTLDR) and ntdetect.com files.
If you do not have an ERD, the emergency repair process can attempt to locate your Windows installation and start repairing your system, but it may not be able to do so.
To run the process, boot from the Windows bootable disks or CD, and choose the Repair option when the system suggests proceeding with an installation or repair. Then press R to run the Emergency Repair Process and choose the Fast or Manual Repair options. Fast Repair is recommended for most users, Manual Repair - for Administrators and advanced users only.
If the emergency repair process is successful, your computer will automatically restart and you should have a working system.
Recovery Console is a command line utility similar to the MS-DOS command line. The following can be preformed: list and display folder content, copy, delete, replace files, format drives and perform many other administrative tasks.
To run Recovery Console, boot from the Windows bootable disks or CD and choose the Repair option. When the system suggests proceeding with installation or repair, press C to run Recovery Console. A prompt will appear asking which system to log on to as-well-as the Administrator's password. After logging on — the drive's contents can be displayed, the existence and safety of critical files can be checked, and have the ability to copy them back if they have been accidentally deleted.
Third party recovery software, in most cases, does not allow you to deal with system files due to the risk of further damage to the system. However, the software can used to check for the existence and safety of these files or to perform virtual partition recovery.