Microsoft

Get IT Done: Safe Mode and Sfc.exe smooth Windows 2000 crash recovery

Troubleshoot boot problems more effectively with Safe Mode and Sfc.exe.


Recovering from a system crash is never an enjoyable experience, but Windows 2000 goes a long way to lessen the pain. The latest version of Windows Safe Mode allows you to troubleshoot boot problems more effectively than ever before, and Sfc.exe finally allows you to restore protected system files. Read on to find out how these two handy tools can be real lifesavers.

Warning
The following article involves editing your system registry. Using the Windows Registry Editor incorrectly can cause serious problems requiring the reinstallation of your operating system and possible loss of data. TechRepublic does not and will not support problems that arise from editing your registry. Use the Registry Editor and the following directions at your own risk.

Working in safe mode
If there is a problem with a driver or some other glitch preventing Windows 2000 from booting normally, Windows can automatically boot in Safe Mode. You may even want to manually boot Windows 2000 to Safe Mode.

For example, say you install a video driver that's buggy or isn't compatible with Windows 2000, and you can't see the desktop properly to log on. Boot to Safe Mode (which loads a standard VGA driver), remove the other driver, and restart. When the system begins to boot, press [F8] in order to display the boot menu, which offers the Safe Mode options.

You can find a list of services that Windows 2000 starts in Safe Mode in this key:
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Safeboot

The Minimal key lists the services that Windows 2000 starts if you select the Safe Mode or Safe Mode With Command Prompt boot options. The Networking key lists the services that start when you select Safe Mode With Networking from the boot menu. Although it's possible to modify these keys to add or remove services, it's not advisable. The service you add could eventually turn out to be the reason why Windows 2000 doesn't start.

You also need to know that Safe Mode sets an environment variable named SAFEBOOT_OPTION, which specifies the current Safe Mode. You can use this environment in a batch file or script to perform tasks based on the mode in which you start Windows. If you boot using Safe Mode or Safe Mode With Command Prompt, the variable is set to Minimal. If you start Windows with Start Mode With Networking, the variable is set to Network.

Use Sfc.exe to restore protected system files
Operating systems prior to Windows 2000 don't provide protection for shared system files such as dynamic link libraries (DLLs). That makes it possible for an application to install an older version of a DLL or executable and cause hung applications or problems with the OS. Windows 2000, on the other hand, provides a level of protection against system changes through the Windows File Protection (WFP) feature. WFP maintains a signature catalog of protected files. WFP checks the file's version when a protected file is modified (typically through an application installation). If the file isn't the correct version, WFP restores the correct file from the Dllcache folder or distribution media as needed, prompting you to provide the source file (such as the Windows 2000 CD).

At the completion of the GUI-mode portion of Setup, Windows 2000 uses a tool called System File Checker (Sfc.exe) to scan all protected system files and verify the existence of the appropriate signature catalog files. If a catalog is corrupted or doesn't exist, SFC restores the catalog from the cache or from the distribution media, again prompting you for the CD if necessary.

Use SFC at any time to scan the system for protected system file changes and to re-create the Dllcache folder if it's damaged or corrupted. Here are several specific SFC commands:
  • SFC /SCANONCE scans the files a single time and repairs them if it's required.
  • SFC /SCANBOOT scans the files each time you boot the system.
  • SFC /CANCEL cancels all pending scans.
  • SFC /? allows you to see additional parameters you can use with SFC.

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