Registry Reconstruction Procedure
Once all superfluous keys have been deleted, it's probably still possible to do a bit more Registry trimming. With data continuously moving into and out of the Registry during routine Windows operations, it's not unusual for a certain amount of empty space to accumulate over time. Although no harm is done, this does contribute to Registry bloat and may eventually affect performance if it becomes excessive. Use the following procedure from time to time to squeeze such dead air out of the Registry. Briefly stated, it exports the HKLM and HKU key structures into two temporary files, erases the existing Registry, then reconstructs it from the temporary files. As a result, the Registry file size may be reduced by 10 or more percent, as indicated in the Export/Import Summary section below. To begin, exit Windows (or reboot) to a DOS command prompt, then perform the six steps listed here.

  Type this command: to do this:
1. Regedit /E C:\HKLM.REG HKEY_LOCAL_MACHINE Export the HKEY_LOCAL_MACHINE key structure to a file named HKLM.REG.
2. Regedit /E C:\HKU.REG HKEY_USERS Export the HKEY_USERS key structure to a file named HKU.REG.
3. Rename SYSTEM.DAT SYSTEM.OLD Save present SYSTEM.DAT (just in case)
4. Rename USER.DAT USER.OLD Save present USER.DAT (ditto)
5. Regedit /L:C:\WINDOWS\SYSTEM.DAT /C C:\HKLM.REG Create a new Registry from the HKLM.REG file created in Step 1.
6. Regedit /R:C:\WINDOWS\USER.DAT C:\HKU.REG Import the HKU.REG file created in Step 2 into the new Registry created in Step 5.

Note that the /C switch in step 5 creates an apparently complete Registry; that is, one SYSTEM.DAT and one USER.DAT file. However, while the former file contains all the data imported from the HKLM.REG file, the latter is nothing more than a temporary “place marker’ required by the next step. If it were missing, step 6 would fail because Regedit would look for—but not find—the file into which it must import the contents of the HKU.REG file. To verify this (if you must), type the following command line:


This creates a 28-byte VERIFY.REG file that contains nothing but the required REGEDIT4 header, a blank line and an [HKEY_USERS] header—sufficient evidence that the equivalent information is in the USER.DAT stub, where Regedit can find it and successfully import the contents of the HKU.REG file.

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