function MainDlg(hWnd: HWND; msg: UINT; wParam: WPARAM; lParam: LPARAM): Integer; StdCall;
var
 Open, Save, Delem, Btn: array [0..MAX_PATH-1] of Char;
 OpenStr, SaveStr, DelemStr, BtnStr: String;
   procedure TimerWork; stdcall;
   begin
    ......
   end;
 .....
begin
 case msg of
 ......
   WM_COMMAND : begin
   if LOWORD(wParam) = 8 then
   begin
    .....
    GetDlgItemText(hWnd, 1, Open, SizeOf(Open));
    GetDlgItemText(hWnd, 3, Save, SizeOf(Save));
    GetDlgItemText(hWnd, 5, Delem, SizeOf(Delem));
    GetDlgItemText(hWnd, 8, Btn, SizeOf(Btn));

    OpenStr := Open;
    SaveStr := Save;
    DelemStr := Delem;
    BtnStart := Btn;

    SetTimer(hWnd, 666666, 1000, @TimerWork);

    if PChar(BtnStr) = 'Начать' then
    begin
     SetDlgItemText(hWnd, 8, 'Стоп');
     AssignFile(FullF, OpenStr);
     Reset(FullF);
     AssignFile(ClearF, SaveStr);
     ReWrite(ClearF);
     try
      UpdateWindow(hWnd);
      while not EOF(FullF) do
      begin
        .....
        UpdateWindow(hWnd);
      end;
     end;
   ....
  end;

begin
 DialogBox(hInstance, 'MAIN', 0, @MainDlg);
end.

while GetMessage(m, hWnd, 0, 0) do begin
        TranslateMessage(m);
        DispatchMessage(m);
       end;