EM_SETSTATE
  The EM_GETSTATE and EM_SETSTATE messages were added so that you may
query and set the internal state flags of an edit control. The only
internal state that you should play around with is STATE_INSERT (defined
in window.h), which controls the overstrike/insert state of a control.

Parameters
  wParam is the new state
  lParam is ignored.
Returns
  Nothing.

Example :

    Turn off the insert flag only
  WORD fEditFlags = SendMessage(hEdit, EM_GETSTATE, 0, 0L);
  SendMessage(hEdit, EM_SETSTATE, fEditFlags & ~STATE_INSERT, 0L);