In XP and up, registry entries have permissions. These can get corrupted in many ways, including by malware and by antivirus tools. If you get a “permission denied” when trying to change or delete a registry entry, you will need to right-click on the folder and add permissions, and then do the deletion. It is usually most straightforward to add the permission to the user logged in, though you’ll have to play with the dialog box if you’re on a domain.