OleDbConnection con = new OleDbConnection(strDbConnString);
con.Open();

OleDbCommand cmd = new OleDbCommand();
cmd.Connection = con;
cmd.CommandText = "SELECT COUNT(*) FROM login WHERE Email=@ParamEmail";
cmd.Parameters.Add("@ParamEmail", TbxEmail.Text);
object oResult = cmd.ExecuteScalar();

string lldt = DateTime.Now.ToString();
cmd.CommandText = "UPDATE login SET LastLogin=@ParamDate WHERE Email=@ParamEmail";
cmd.Parameters.Add("@ParamDate", lldt);
cmd.Parameters.Add("@ParamEmail", TbxEmail.Text);
cmd.ExecuteNonQuery();
con.Close();

cmd.Dispose();
con.Dispose();