Registry DeleteEx is a portable tool which can delete locked Registry keys, even if they are owned by Windows or protected by malware.
The interface is basic in the extreme. There's no REGEDIT-type viewer, no tree to help you find the key you need: just boxes to enter a key or value, and a Delete button to get rid of it.
What makes Registry DeleteEx interesting is that it ignores the usual Windows deletion method, and writes to the Registry directly. As a result, the program bypasses any permissions issues, and it won't matter if malware has hooked the Windows API: the key is deleted anyway. (Of course, if malware is running on your PC, then it can always rewrite the key later, but Registry DeleteEx at least gives you a chance of regaining control.)
Beware: if you delete something important then it could crash and/ or cripple your PC. Only use this program if you know what you're doing, and how you can recover if it all goes horribly wrong.