How do I clear unwanted entries from Explorer's

Run the Registry Editor (Start - Run - C:\WINDOWS\REGEDIT.EXE),
Select Find from the Edit menu, type ShellNew, and press OK,
Every ShellNew that is found will be a branch of a particular file type - If that file type is not wanted in the New menu, delete the entire ShellNew branch.
Note: New templates can also be removed (and created) with TweakUI,
one of Microsoft's PowerToys