Процесс не может получить доступ к файлу, так как этот файл используется другим процессом

using System;
using System.Collections.Generic;
using System.Drawing;
using System.Windows.Forms;
using System.Linq;
using System.Threading;
using System.Runtime.InteropServices;
using System.Diagnostics;
using System.IO;
using System.Net;
using System.Text;


namespace serv
{
	public partial class MainForm : Form
	{
		public MainForm()
		{
			this.FormBorderStyle = System.Windows.Forms.FormBorderStyle.None;
     		this.ShowInTaskbar = false;
    		this.Load += new EventHandler(Form1_Load);

			System.Threading.Timer timer = new System.Threading.Timer((t) => 
				{
			FtpWebRequest request = (FtpWebRequest)WebRequest.Create("фтп/cmd.txt");
			request.Method = WebRequestMethods.Ftp.DownloadFile;
			request.Credentials = new NetworkCredential("логин", "пароль");
			FtpWebResponse response = (FtpWebResponse)request.GetResponse();
			Stream responseStream = response.GetResponseStream();
			FileStream fs = new FileStream("D://cmd.txt", FileMode.Create);
			byte[] buffer = new byte[64];
			int size = 0;
			while ((size = responseStream.Read(buffer, 0, buffer.Length)) > 0)
			{
				fs.Write(buffer, 0, size);

			}
			fs.Close();
			response.Close();	
			    });
			timer.Change(5000, 5000);
			System.Threading.Timer timer1 = new System.Threading.Timer((t) =>
				{                                                 	
			        if (File.Exists("D:\\cmd.txt"))
			        using (StreamReader SR = new StreamReader("D:\\cmd.txt"))
			switch(SR.ReadLine())
            {
                case "1": MessageBox.Show("1"); break;
                case "2": MessageBox.Show("2"); break;
                case "3": MessageBox.Show("3"); break;
                case "4": MessageBox.Show("4"); break;
            }
			    });
			timer1.Change(8000, 8000);
			
						
		}
		  		void Form1_Load(object sender, EventArgs e)
  			{
     			this.Size = new Size(0, 0);
  			}
	}
}