//whenever check box is checked, you need to execute below code lines:

HttpCookie aCookie = new HttpCookie("userInfo");
aCookie["userName"] = login.LstrLoginName ;
aCookie["password"] = login.LstrPassword ;
HttpContext.Current.Response.Cookies.Add(aCookie);


//Next time, whenever Login page loads, you need to execute below code lines:

HttpCookie aCookie = Request.Cookies["userInfo"];
txtuserName.text = aCookie["userName"];
txtPWD.text = aCookie["password"];