When you attempt to start the Registry Editor (regedit.exe) in your Windows Vista system, you may receive the following message:
Registry editing has been disabled by your administrator

If the above error message is shown in a standalone system, then it may be a result of Malware infection. It is also possible that the DisableRegistryTools policy had been enabled by a third-party tweak program which you may be using. It's highly advisable to follow up with a complete system checkup for Virus and Malware, especially when you're unsure about the cause.
If you are using an edition of Windows Vista (Windows Vista Business, Enterprise and Ultimate editions) which includes the Group Policy Editor, follow these steps:
User Configuration | Administrative Templates | System
Important: If it's already set to Not Configured, set it to Enabled and click Apply. Then repeat the steps and revert the setting back to Not Configured. This removes the registry based policy if set already.
This method should work in all editions of Windows Vista.
cd /d %userprofile%\desktop
wscript.exe reg_enable.vbs
If everything worked fine, the DisableRegistryTools policy should have been removed by now. See if you're able to launch Regedit.exe now.