Editing Explorer Drop-down Menu

When you open Explorer and right-click on a folder or file, you get a drop-down menu which contains a number of functions such as Search or Properties, etc.
I'd like to remove an item placed there by one of my utility programs and also would like to add a function such as Defrag to allow Defragmenting individual Folders or Files (specifically the Restore folder under the "System Volume Information" folder.
I can't find where this particular "drop-down" menu exists in the registry.
Any advice in this area would be appreciated.