Remember My Password At The Time Of Login

Put a CheckBox on your form to control whether or not the user wants to remember the password. If this is checked, store the username and password in a cookie. This is the cookie that you will be checking when the user comes back to your application later on. For bonus points, put an expiration date in the cookie so that the user will be required to log in again if they don't come back to your site for some period of time - 1 month seems a reasonable value.

Here you go ,

Step1:-
In aspx add these

<tr>
                        <td width="12%" align="right"><input id="chkbxRempasss" type="checkbox" runat="server" name="checkbox" value="checkbox" /></td>
                        <td width="88%"><span class="table-col2_7">Remember my password </span></td>
                      </tr>

Step2:-
In Aspx.cs you can write something like this to store it in cookies

public partial class Login : System.Web.UI.Page
    {
        string username = string.Empty;
        string pwd = string.Empty;
        string encytpwd = string.Empty;
        string UserID = string.Empty;
        ProcessService ProcessService = new ProcessService();

        protected void Page_Load(object sender, EventArgs e)
        {
            if (Request.Cookies["myCookie"] != null)       
            {            
                HttpCookie cookie = Request.Cookies.Get("myCookie");            
                //txtUserName.Value = cookie.Values["username"];            
                txtUserName.Attributes.Add("value", cookie.Values["username"]);
                //txtPwd.Value = cookie.Values["password"];            
                txtPwd.Attributes.Add("value", cookie.Values["password"]);

                Response.Cookies["myCookie"].Expires = DateTime.Now.AddDays(-1);            
                //Response.Cookies[Txt_Password.Text].Expires = DateTime.Now.AddDays(-1);
                chkbxRempasss.Checked = true;
            }

            if (Page.Request.Params.Get("__EVENTTARGET") != null)
            {
                ChkUserLogin();
            }
        }
//Check Login Credentials
        protected void ChkUserLogin()
        {
            try
            {
                HttpCookie myCookie = new HttpCookie("myCookie"); 
                if (chkbxRempasss.Checked == true) 
                { 
                    myCookie.Values.Add("username", txtUserName.Text.ToString()); 
                    myCookie.Values.Add("password", txtPwd.Text.ToString()); 
                    myCookie.Expires = DateTime.Now.AddDays(15); 
                    Response.Cookies.Add(myCookie); 
                }

                username = txtUserName.Text.ToString();
                pwd = txtPwd.Text.ToString();
                encytpwd = Encrypt(pwd, "&%#@?,:*");
                string result = string.Empty;
                result = ProcessService.ChkUserLogin(username, encytpwd);
                if (result == "0")
                {
                    DataSet User = ProcessService.getLoginuserdetails(username);
                    UserID = User.Tables[0].Rows[0][0].ToString();
                    Session["UserID"] = UserID.ToString();
                    Response.Redirect("Default.aspx");
                }
                else if (result == "1")
                {
                    lblAlert.Text = "Invalid UserName.";
                    lblAlert.ForeColor = System.Drawing.Color.Red;
                    lblAlert.Visible = true;
                }
                else if (result == "2")
                {
                    lblAlert.Text = "Invalid Password.";
                    lblAlert.ForeColor = System.Drawing.Color.Red;
                    lblAlert.Visible = true;
                }
            }
            catch (Exception exn)
            {
            }
            finally
            {
            }
        }

All the best:) write back if you need further support!