To remove a context menu handler, open the Run dialog by pressing Windows + R, type “regedit,” and press Enter. Navigate to HKEY_CLASSES_ROOT\shellex\ContextMenuHandlers.

Have you tried looking with ShellExView?<BR><BR>Also, there's this article discussing the problem. It lists some places for context handlers that I did not know.

Windows only: ShellNewHandler is a small, installation-free tool that cleans up and eliminates programs and files from your your Windows context menu' ...