private void btnOK_Click(object sender, EventArgs e)
{
    if (ValidLogin(txtUsername.Text.Trim(), txtPassword.Text.Trim()))
    {
        FrmMain f = new FrmMain(this);
        f.Show();
        this.Hide();
    }
    else
    {
        MessageBox.Show("错误", this.Text, MessageBoxButtons.OK, MessageBoxIcon.Exclamation);
    }
}

private bool ValidLogin(string username, string password)
{
    //数据库放到项目目录/bin/debug 目录下
    bool result = false;
    OleDbConnection cn = new OleDbConnection("Provider=Microsoft.Jet.OLEDB.4.0;Data Source=your.mdb");
    string strSql = "select 1 from Users where uid = @uid and pwd = @pwd";
    OleDbCommand cmd = new OleDbCommand(strSql, cn);
    cmd.Parameters.AddWithValue("@uid", txtUsername.Text);
    cmd.Parameters.AddWithValue("@pwd", txtPassword.Text);

    cn.Open();
    OleDbDataReader dr = cmd.ExecuteReader();
    if (dr.HasRows)
    {
        result = true;
    }
    cn.Close();

    return result;
}

private void btnCancel_Click(object sender, EventArgs e)
{
    this.Close();
}