Registry Editing has been disabled by your administrator

Removing the DisableRegistryTools restriction

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:

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
• Or Prevent access to registry editing tools and set it to Not Configured
• Exit the Group Policy Editor

http://windowsxp.mvps.org/tweakuirest.htm