The REGISTRY EDITOR is dissable by adminstrator:-
Registry Editing has been disabled by your administrator For standalone Windows XP systems, perform the steps below to remove the registry editing restrictions. 
Method 1: Using the REG.EXE console tool
1. Click Start, Run and type this command: 
REG add HKCU\Software\Microsoft\Windows\CurrentVersion\Policies\System /v DisableRegistryTools /t REG_DWORD /d 0 /f
(Just Copy and paste the above Command in RUN)You should be able to launch Tweak UI, as well as the Registry Editor. 
Method 2: Using the Group Policy Editor (Windows XP Professional only)
Click Start, Run and type gpedit.msc and press ENTER Go to the following location: 
User Configuration Administrative Templates System 
Double-click Disable registry editing tools and set it to Not Configured 
Exit the Group Policy Editor Note: If the setting already reads Not Configured, set it to Enabled, and click Apply. Then revert it back to Not Configured. This ensures that the DisableRegistryTools registry value is removed successfully.