How to check password is correct or not.

ASP PAGE CODE

<table class="style1">
            <tr>
                <td class="style2">
                    <asp:Label ID="Label1" runat="server" Text="USERNAME"></asp:Label>
                </td>
                <td class="style3">
                    <asp:TextBox ID="txtUserName" runat="server" BorderStyle="Solid" 
                        BorderWidth="1px" Height="18px" Width="127px"></asp:TextBox>
                </td>
            </tr>
            <tr>
                <td class="style2">
                    <asp:Label ID="Label2" runat="server" Text="PASSWORD"></asp:Label>
                </td>
                <td class="style3">
                    <asp:TextBox ID="txtPassword" runat="server" BorderStyle="Solid" 
                        BorderWidth="1px" Height="18px" TextMode="Password" Width="127px"></asp:TextBox>
                </td>
            </tr>
            <tr>
                <td colspan="2" style="text-align: center">
                    <asp:Button ID="btnLogin" runat="server" Text="Login" BackColor="White" 
                        BorderStyle="Solid" BorderWidth="1px" onclick="btnLogin_Click" />
                </td>
            </tr>
            <tr>
                <td colspan="2" style="text-align: center">
                    <asp:Label ID="lbStatus" runat="server"></asp:Label></td>
            </tr>
        </table>

BACK END CODE C#

    protected void btnLogin_Click(object sender, EventArgs e)
    {
        SqlConnection con = new SqlConnection();
        con.ConnectionString = @"Data Source=.\SQLEXPRESS;AttachDbFilename=|DataDirectory|\ASPNETDB.MDF;Integrated Security=True;User Instance=True";
        SqlCommand cmd = new SqlCommand();
        con.Open();
        cmd.CommandText = @"SELECT USERNAME FROM USERS_TABLE WHERE USERNAME='"+txtUserName.Text+"' AND PASSWORD='"+txtPassword.Text+"'";
        cmd.Connection = con;
        object o = cmd.ExecuteScalar();
        if (o == null)
        {
            lbStatus.Text = "Invalid login";
            return;
        }
        Session["username"] = o;
        lbStatus.Text = "Success";
        con.Close();
    }

Thanks King,
but i know about the connectivity about sql server but i need to check os password
suppose i have one application and application required os password so at that time how to check whether that password is correct or not


Thanks And Regards.
Kamlesh Nikam

http://stackoverflow.com/questions/3714679/how-get-current-username-and-password-from-windows-operating-system

Thanks And Regards.
Kamlesh Nikam