REGEDIT

Import, export or delete registry settings from a text (.REG) file

Syntax
      Export the Registry (all HKLM plus current user)
      REGEDIT /E pathname

      Export part of the Registry
      REGEDIT /E pathname "RegPath"

      Import a reg script
      REGEDIT pathname

      Silent import
      REGEDIT /S pathname

      Start the regedit GUI
      REGEDIT 

      Open multiple copies of GUI (XP and 2003 only)
      REGEDIT -m

Key
   /E : Export 

   /S : Silent Import

How to add keys and values from the registry:

Create a text file like this:

Windows Registry Editor Version 5.00 
[HKEY_CURRENT_USER\SomeKey]
"SomeStringValue"="Hello" 

When double clicking this .reg file the key and value will be added.
Alternatively run REGEDIT MYKEY.REG from the command line.

How to delete keys and values from the registry:

Create a reg file like this, notice the hyphen inside the first bracket

Windows Registry Editor Version 5.00 
[-HKEY_CURRENT_USER\SomeKey]  

When double clicking this .reg file the key "SomeKey" will be deleted along with all string, binary or Dword values in that key.

If you want to just delete values, leaving the key in place, set the value you want to delete = to a hyphen
e.g.

Windows Registry Editor Version 5.00 
[HKEY_CURRENT_USER\SomeKey]
"SomeStringValue"=-

Again double clicking this .reg file will delete the values specified, or you can use REGEDIT /s MyDeleteScript.REG

Compare the Registry of two machines

Windiff is your friend, this simple GUI utility from the resource kit will list all the differences.

Comments

Within a registry file, comments can be preceded by "; "
e.g.
;
; Turn the NUMLOCK on at login
;
[HKEY_CURRENT_USER\Control Panel\Keyboard]
"InitialKeyboardIndicators"="2"

Under Windows NT 4 all registry scripts start with: REGEDIT4
(This version string will also work in XP and later versions of Windows.)

"I never make stupid mistakes. Only very, very clever ones" - John Peel

Related:

REG - Read, Set or Delete registry keys and values
SET - Display, set, or remove Windows NT environment variables
SETX - Set environment variables permanently
WMIC REGISTRY - Set registry options through WMI
Powershell: Get-ItemProperty / Set-ItemProperty - Retrieve / save registry Keys
Q322756 - How to backup and edit the registry
Dureg - Registry Size Estimator. (Win 2K ResKit)
XP Registry Keys - Commonly tweaked user interface settings