Registry Editing has been disabled by your administrator

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
Exit the Group Policy Editor