how to check validate account before approve new account

Hi,

how can i check user account isvalid or not befor approve. i have use below code but user account always false.

Code:

MembershipUser user = Membership.GetUser(userId);//user id will get from query string
if (Membership.ValidateUser(txtUserName.Text.Trim(), txtPassword.Text.Trim()))// here i'm getting always false
    {
     user.IsApproved = true;
     Membership.UpdateUser(user);
     lblMessage.Text = "Congrats! your account has been approved";
     }
else
    {
   lblMessage.Text = "invalied Details";
    }

Appreciate for Best and Quick response.