private void Form1_Load(object sender, EventArgs e)
        {
            string strConn = "server=.;user id=dbo;pwd=;database=admin";
            SqlConnection con = new SqlConnection(strConn);
            string sql = "select * from adminpswd";
            SqlDataAdapter ada = new SqlDataAdapter(sql, con);
            DataSet ds = new DataSet();
            ada.Fill(ds);

        }
        private void button1_Click(object sender, EventArgs e)
        {
         
            if (this.textBox1.Text == "")
            {
                MessageBox.Show("用户名不能为空！", "错误");
            }
            else if (this.maskedTextBox1.Text == "")
            {
                MessageBox.Show("密码不能为空！", "错误");
            }
            else
            {
                if (IsUser(this.textBox1.Text))
                {
                    if (this.maskedTextBox1.Text == LoginUser(this.textBox1.Text))
                    {
                        Form2 frm2 = new Form2();
                        // //frm2.showdialog();
                        frm2.Show();
                        Close();
                    }
                    else
                    {
                        MessageBox.Show("密码错误！");
                    }
                }
                else
                {
                    MessageBox.Show("用户名错误！");
                }
            }

        }

        private string LoginUser(string uName) //检测用户是否能够登录
        {
            DataSet ds = new DataSet();
            return ds.adminpswd[0].Rows[0]["密码"].ToString();
            con.Close();
        }


        private bool IsUser(string uName) //检测用户输入的用户名是否存在
        {
            DataSet ds = new DataSet();
            ada.Fill(ds, "adminpswd");

            int n;
            n = ds.adminpswd[0].Rows.Count;
            if (n > 0)
                return true;//存在就返回true
            else
                return false;

            con.Close();
        }

        private void button2_Click(object sender, EventArgs e)//button2的 Text属性是:取消登录
        {
            Application.Exit();//用户取消登录，退出系统
        }

说是，系统知不道adminpswd...

LZ,你的代码中好多都是定义了局部变量,外部函数怎能访问这些变量呢,注译如下:
private void Form1_Load(object sender, EventArgs e)
        {
            string strConn = "server=.;user id=dbo;pwd=;database=admin";
            SqlConnection con = new SqlConnection(strConn);
            string sql = "select * from adminpswd";
            SqlDataAdapter ada = new SqlDataAdapter(sql, con);
            DataSet ds = new DataSet();
            ada.Fill(ds);

        }
        private void button1_Click(object sender, EventArgs e)
        {
         
            if (this.textBox1.Text == "")
            {
                MessageBox.Show("用户名不能为空！", "错误");
            }
            else if (this.maskedTextBox1.Text == "")
            {
                MessageBox.Show("密码不能为空！", "错误");
            }
            else
            {
                if (IsUser(this.textBox1.Text))
                {
                    if (this.maskedTextBox1.Text == LoginUser(this.textBox1.Text))
                    {
                        Form2 frm2 = new Form2();
                        // //frm2.showdialog();
                        frm2.Show();
                        Close();
                    }
                    else
                    {
                        MessageBox.Show("密码错误！");
                    }
                }
                else
                {
                    MessageBox.Show("用户名错误！");
                }
            }

        }

        private string LoginUser(string uName) //检测用户是否能够登录
        {
            DataSet ds = new DataSet();
            return ds.adminpswd[0].Rows[0]["密码"].ToString();         //ds的数据集何来?
            con.Close();                                               //con的变量定义?是Load事件函数的局部变量吗?如果是,就不能这样访问
                                                                       //最后,前一行已使用了"return","con.Close()"就不能执行了
                                                                       //建议:将"con.Close()"放在"return"行前面
        }


        private bool IsUser(string uName) //检测用户输入的用户名是否存在
        {
            DataSet ds = new DataSet();
            ada.Fill(ds, "adminpswd");                            //ada定义何在?如果是Load事件函数中的局部变量,不能这样访问

            int n;
            n = ds.adminpswd[0].Rows.Count;
            if (n > 0)
                return true;//存在就返回true
            else
                return false;

            con.Close();                                           //con的变量定义?是Load事件函数的局部变量吗?如果是,就不能这样访问
                                                                   //上面已经使用了"return","con.Close()"不能再执行了
                                                                   //建议:将"con.Close()"放在If前面
        }

        private void button2_Click(object sender, EventArgs e)//button2的 Text属性是:取消登录
        {
            Application.Exit();//用户取消登录，退出系统
        }

建议:
将con和ada定义为全局变量来测试

试试这个数据库连接
SqlConnection myCon = new SqlConnection();
myCon.ConnectionString = "Persist Security Info=False;User id=dbo;pwd=;                                  database=admin;server=(local)";
myCon.Open();