Visual C++ 6.0
By default, Visual Assist disables DDE to avoid DEVDBG exceptions from Visual C++ 6.0. As a result, the IDE no longer opens header files or source files when you double-click them.
Re-enable DDE by exiting all IDEs and creating the BINARY registry value:
HKCU\Software\Whole Tomato\Visual Assist X\AllowDDE = 01