The registry on newer Windows systems has access control lists. This means that even if you run regedit.exe as an Administrator, you can still get “Access Denied” trying to erase or alter some keys (CurrentControlSet/Enum/USB comes to mind).
The solution to this is to run the registry editor as the “System” account, which can be done using psExec.
In short, the command you want is psexec -i -d -s c:\windows\regedit.exe, and psexec is available as part of PSTools here:
http://technet.microsoft.com/en-us/sysinternals/bb897553.aspx
Copy here, just in case: PSTools