function ActivatePrevInstance(const MainFormClass, ATitle: string): Boolean;
var
  PrevWnd, PopupWnd, ParentWnd: HWnd;
{$IFDEF WIN32}
  IsDelphi: Longint;
{$ELSE}
  S: array[ 0 .. 255 ] of Char;
{$ENDIF}
begin
  Result := False;
  PrevWnd := FindPrevInstance(MainFormClass, ATitle);
  if PrevWnd <>  0  then begin
    ParentWnd := GetWindowParent(PrevWnd);
    while (ParentWnd <> GetDesktopWindow) and (ParentWnd <>  0 ) do begin
      PrevWnd := ParentWnd;
      ParentWnd := GetWindowParent(PrevWnd);
    end;
    if WindowClassName(PrevWnd) = 'TApplication' then begin
{$IFDEF WIN32}
      IsDelphi :=  0 ;
      EnumThreadWindows(GetWindowTask(PrevWnd), @WindowsEnum,
        LPARAM(@IsDelphi));
      if Boolean(IsDelphi) then Exit;
{$ELSE}
      GetModuleFileName(GetWindowTask(PrevWnd), S, SizeOf(S) -  1 );
      if AnsiUpperCase(ExtractFileName(StrPas(S))) = 'DELPHI.EXE' then Exit;
{$ENDIF}
      if IsIconic(PrevWnd) then begin { application is minimized }
        SendMessage(PrevWnd, WM_SYSCOMMAND, SC_RESTORE,  0 );
        Result := True;
        Exit;
      end
      else ShowWinNoAnimate(PrevWnd, SW_SHOWNOACTIVATE);
    end
    else ActivateWindow(PrevWnd);
    PopupWnd := GetLastActivePopup(PrevWnd);
    if (PrevWnd <> PopupWnd) and IsWindowVisible(PopupWnd) and
      IsWindowEnabled(PopupWnd) then
    begin
{$IFDEF WIN32}
      SetForegroundWindow(PopupWnd);
{$ELSE}
      BringWindowToTop(PopupWnd);
{$ENDIF}
    end
    else ActivateWindow(PopupWnd);
    Result := True;
  end;
end;