File Extension question

[Gunner]

Are there any file extensions that should not be deleted from the registry?  Example would be .exe, .dll, .com etc. If these are deleted, would there be any adverse affects on the OS?  If I delete .exe, would the OS still know what to do?  Probably would cause havoc.  I ask because I need to (feel I need to) prevent certain extensions from being deleted (or modified) in my program.  What extensions should I prevent from being deleted or modified? Thanks