Code
<table border="0" width="100%" cellspacing="0" cellpadding="3" height="100">
 <tr>
  <td align="right" width="60">用户名：</td>
  <td align="left">
   <asp:TextBox ID="txtUsername" runat="server" MaxLength="16" Width="160px" Height="24px"></asp:TextBox>
  </td>
 </tr>
 <tr>
  <td align="right" width="60">密码：</td>
  <td align="left">
   <asp:TextBox ID="txtPassword" runat="server" MaxLength="16" TextMode="Password" Height="24px" Width="160px"></asp:TextBox>
  </td>
 </tr>
 <tr>
  <td align="right" width="60">　</td>
  <td align="left">
   <asp:Button ID="btnLogin" runat="server" Text=" 登录 " Height="24px" Width="60px" onclick="btnLogin_Click" />
  </td>
 </tr>
</table>

Code
protected void [color=#FF0000]btnLogin_Click[/color](object sender, EventArgs e)
    {
        if (Admin.CheckAdmin(txtUsername.Text.Trim(), txtPassword.Text.Trim()))
        {
            //验证成功
            Session["usern"] = txtUsername.Text;
            Session["login"] = "true";
            Response.Redirect("manage.aspx");
        }
        else
        {
            //验证失败
            Response.Write("login error.");
        }
    }