procedure ListFileDir(Path: string; FileList: TStrings);
var
   SR: TSearchRec;
begin
   if FindFirst(Path + '*.*', faAnyFile, SR) = 0 then
   begin
     repeat
       if (SR.Attr <> faDirectory) then
       begin
         FileList.Add(SR.Name);
       end;
     until FindNext(SR) <> 0;
     FindClose(SR);
   end;
end;

procedure TForm1.Button1Click(Sender: TObject);
begin
   ListFileDir('C:\WINDOWS\', ListBox1.Items);
end;