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When you start your computer successfully, the Windows Registry Checker tool (Scanreg.exe) creates a backup of system files and registry configuration information (including user account information, protocol bindings, software program settings, and user preferences) once daily. Files that Windows Registry Checker backs up include System.dat, User.dat, System.ini, and Win.ini. This article describes the Windows Registry Checker tool.

More Information

Windows Registry Checker automatically scans the system registry for invalid entries and empty data blocks when it is started. If invalid registry entries are detected, Windows Registry Checker automatically restores a previous day's backup. This is equivalent to running the scanreg /autorun command from a command prompt. If no backups are available, Windows Registry Checker tries to make repairs to the registry. This is equivalent to running the scanreg /fix command from a command prompt. If the registry contains more than 500 KB of empty data blocks, Windows Registry Checker automatically optimizes it.

Windows Setup runs the Windows Registry Checker tool to verify the integrity of the existing registry before it performs an upgrade. If it detects registry damage, it tries to fix it automatically.

The protected-mode version of the Windows Registry Checker tool (Scanregw.exe) can create a backup of the system files and scan the registry for invalid entries. If invalid entries are detected, it refers to the real-mode version of the Windows Registry Checker tool (Scanreg.exe) for a resolution.

You can configure Windows Registry Checker with a Scanreg.ini file. Settings that you can configure include:

  • Enabling or disabling the tool

  • The number of backups maintained (no more than five is recommended)

  • The location of the backup folder

  • Settings to add additional files to the backup set

For additional information about the Scanreg.ini file, click the article number below to view the article in the Microsoft Knowledge Base:

183603 How to Customize Registry Checker Tool Settings To start the Windows Registry Checker tool, click Start, click Run, type scanregw.exe in the Open box, and then click OK.

NOTE: To use the Windows Registry Checker tool with the /restore parameter, you must run the tool from a command prompt running outside of Windows. When you do so, you can choose up to five registry backup files listed for you to restore.

To Restore Individual Files

To restore individual files, follow these steps:

  1. Click Start, point to Find, and then click Files Or Folders.

  2. In the Named box, type rb0*.cab, and then click Find Now.

  3. Double-click the cabinet file that contains the file that you want to restore.

  4. Right-click the file that you want to restore, click Extract, and then choose the folder where the new file is to be placed. Microsoft recommends that you place the file in your Temp folder.

  5. Restart your computer in MS-DOS mode (in Windows Millennium Edition, this requires that you restart with the Windows Millennium Edition Startup disk).

  6. Copy the file that you extracted to the appropriate folder. Note that registry .dat files are typically marked as hidden and read-only, so you need to use both the attrib and copy commands to replace the existing file with the newly extracted one.

Known Issues for Windows Registry Checker

If your registry contains an entry that references a file (such as a .vxd file) that no longer exists, it is not repaired by Windows Registry Checker. Such errors are not typically damaging, and you can manually remove the entry.

The amount of conventional memory that is required by Windows Registry Checker is determined by the size of your registry. Windows Registry Checker may require 580 KB or more of free conventional memory to complete the repair process. If you encounter an "Out of Memory" error message, optimize your free conventional memory.

NOTE: Extended memory is required for Windows Registry Checker to operate properly, so it does not run when you start your computer with the Safe Mode Command Prompt Only option. The exception to this is the scanreg /restore command, which is the only Scanreg function that can run without extended memory memory.


For additional information about Registry Checker command line switches, click the article number below to view the article in the Microsoft Knowledge Base:

184023 Command-Line Switches for the Registry Checker Tool

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