Major System Files, and their purpose: Windows 9x –specific files IO.SYS The io.sys file is an MS-DOS and Windows 9x hidden system file that is used to load the operating system each time the computer boots. The file is not editable using normal text editors. MSDOS.SYS The MSDOS.SYS file is a hidden system file created on the root of the boot drive. To edit this file, type edit msdos.sys. The PC boot environment can be changed by changing the file contents. Given below is a typicalMSDOS.SYS file: Below is a sample MSDOS.SYS File [Paths] WinDir=C:\WINDOWS WinBootDir=C:\WINDOWS HostWinBootDrv=C [Options] BootGUI=1 Network=1 BootMulti=1 ; ;The following lines are required for compatibility with other programs. ;Do not remove them (MSDOS.SYS needs to be >1024 bytes). AUTOEXEC.BAT An AUTOEXEC.BAT file contains DOS commands that are executed automatically when a PC boots. The file is usually located in the root directory of the hard drive of the computer. The AUTOEXEC.BAT file is used to set defaults and to run programs that should be executed during startup. Example: PROMPT $P$G Use the PROMPT command to alter the default DOS prompt. The above command makes the DOS prompt display the current path and drive, and is very useful. COMMAND.COM 1. It does: The file contains internal command set and error messages 2. Default Attributes: Nil 3. Is it required for OS Start up: YES 4. Responsible for displaying the command prompt in a DOS based computer. CONFIG.SYS 1. It does: Loads low level device drivers and does performance tuning 2. Default Attributes: Nil 3. Is it required for OS Start up: NO EMM386.exe EMM386.EXE allows access to Upper Memory Area. Please note that the conventional memory of 1 MB is divided into 1. Lower Memory Area 640 KB, and 2. Upper Memory Area 384KB (1024KB-640KB). HIMEM.SYS 1.HIMEM.SYS must be loaded before EMM386.EXE 2.HIMEM.SYS is used to address the extended memory WIN.COM The executable file located in the windows directory used to load Microsoft Windows. SYSTEM.INI Used with Microsoft Windows 9x to initialize system settings for the computer, such as the fonts, keyboard, language and other settings. The system.ini file is located in the C:\windows directory. Users looking to view or edit the system.ini file can use the sysedit or msconfig utilities. Windows NT 4.0, 2000, XP, or later operating systems do not have a system.ini. Instead, the initialization setting for these OSes are stored in the system registry. WIN.INI The Microsoft Windows 3.x and 9x initialization file located in the C:\Windows directory. The win.ini file is used to load various settings each time Windows boots; for example, the communications drivers, wallpaper, screensaver, languages, fonts, etc... are loaded each time the win.ini is initialized. If this file becomes corrupt or bad, Windows will either not load, or have several errors as it loads. Users looking to view or edit the system.ini file can use the sysedit or msconfig utilities. Users who are running Windows NT 4.0, 2000, XP, or later operating systems no longer have a win.ini. Instead of storing these settings in this file, they are stored in the system registry. Registry data files SYSTEM.DAT USER.DAT SYSTEM.DAT, USER.DAT files are part of windows Registry and can be edited using REGEDIT or REGDT32 utility. Registry files can't be read with standard text editors. Further, USER.DAT file corresponds to HKEY_LOCAL_USER and SYSTEM.DAT corresponds to HKEY_LOCAL_MACHINE. Windows NT-based specific files The following files are used by Windows NT/2000/XP Operating Systems. NTLDR (NTLoaDeR) It is a program loaded from the hard drive when the system starts up. The NTLDR launches Windows NT/2000 or other OS subsequently. NTLDR displays the boot options by reading from BOOT.INI file. BOOT.INI This is a text file that contains OSs available on the computer. The contents are fetched by NTLDR and displayed as OS options. An example is given below. [boot loader] timeout=10 default=multi(0)disk(0)rdisk(0)partition(2)\WINDOWS [operating systems] multi(0)disk(0)rdisk(0)partition(2)\WINDOWS="Microsoft Windows XP" /fastdetect C:\ = "Microsoft Windows" NTDETECT.COM The program detects the installed hardware on the system. NTBOOTDD.SYS This file is used to provide access to SCSI boot partition. If the name of the path in Boot.ini uses SCSI syntax, the Ntbootdd.sys must be in the root folder of the system partition. NTUSER.DAT Ntuser.dat is stored on the local computer at: %SystemRoot%\Profiles\Default User, and contains the default user profile. When a new user is created, the user is assigned default properties as defined in ntuser.dat. One way to change the default user settings is to edit ntuser.dat using registry editor ntedt32.exe For example, if you want to set the default language to some other language instead of the English, you can edit this file to correspond to the new language. .