Here are a few links that found with the basic and not so basic about context menu editing in the registry: Manage the context-menu entries for files, folders, drives and Namespace objects How to Clean Up Your Messy Windows Context Menu - How-To Geek Customize the Right Click Menu in Windows 7
While some may label as WinXP, it really applies to Vista, 7 and 8 too, and part of it to earlier versions too.
In XP, the context menu editor was in: Windows Explorer => Tools menu => Folder options => File Types tab.
The very same dialog still exist on Windows 7, but the file types tabs has been removed. The "Default Programs" control panel applet is a poor substitute of the full fledged dialog, and if you go directly to the registry, you can do a few more things that even the dialog can't handle.
Just take caution when toying with the registry and take a backup or restore point, just in case.