If your system is in a Network, you may need to contact the Network Administrator to relax this restriction (if needed). For standalone PCs, try the following and remove the registry editing restrictions:
1. Click Start, Run and type this command exactly as given below: (better - Copy and paste)
REG add HKCU\Software\Microsoft\Windows\CurrentVersion\Policies\System /v DisableRegistryTools /t REG_DWORD /d 0
Now, you should be able to launch TweakUI, as well as the Registry Editor.
Using the Group Policy Editor - For Windows XP Professional
DisableRegistryTools value corresponds to the Disable registry editing tools Group Policy
User Configuration \ Administrative Templates \ System
Click Start, Run and type GPEDIT.MSC to launch the Group Policy Editor, and set the policy to Not Configured