Here is my XML Definition
<?xml version="1.0" encoding="utf-8" ?>
<hibernate-mapping xmlns="urn:nhibernate-mapping-2.0"
                   namespace="NHibernateASPSample.Domain"
                   assembly="NHibernateASPSample.Domain">

  <class name="DbUser" table="DbUser">
    <id name="IdUser" column="IdUser" type="Int32">
      <generator class="identity"/>
    </id>
    <property name="Login"    column="Login"    type="String"/>
    <property name="Password" column="Password" type="String"/>
    <property name="UserData" column="UserData" type="String"/>
   
  </class>

</hibernate-mapping>

using System;
using System.Web;


namespace NHibernateASPSample.Domain
{
    /// <summary>
    /// Class that holds the data of the table DbUser
    /// </summary>
    public class DbUser
    {
        #region Members

        private int _idUser;
        private string _login;
        private string _password;
        private string _userData;

        #endregion

        /// <summary>
        /// A standard-constructor is needed!
        /// </summary>
        public DbUser()
        {
        }

        public int IdUser
        {
            get { return _idUser; }
            set { _idUser = value; }
        }

        public string Login
        {
            get { return _login; }
            set { _login = value; }
        }

        public string Password
        {
            get { return _password; }
            set { _password = value; }
        }

        public string UserData
        {
            get { return _userData; }
            set { _userData = value; }
        }
    }
}

<?xml version="1.0"?>
<!--
    Note: As an alternative to hand editing this file you can use the
    web admin tool to configure settings for your application. Use
    the Website->Asp.Net Configuration option in Visual Studio.
    A full list of settings and comments can be found in
    machine.config.comments usually located in
    \Windows\Microsoft.Net\Framework\v2.x\Config
-->
<configuration>
   <configSections>
      <section name="nhibernate" type="System.Configuration.NameValueSectionHandler, System,Version=1.0.5000.0,Culture=neutral,PublicKeyToken=b77a5c561934e089"/>
   </configSections>
   <nhibernate>
      <add key="hibernate.connection.provider" value="NHibernate.Connection.DriverConnectionProvider"/>
      <add key="hibernate.dialect" value="NHibernate.Dialect.MsSql2000Dialect"/>
      <add key="hibernate.connection.driver_class" value="NHibernate.Driver.SqlClientDriver"/>
      <add key="hibernate.connection.connection_string" value="User ID=sa;Password=sa1267;Data Source=GENESIS200;Initial Catalog=BestRetailSolution"/>
   </nhibernate>
   <appSettings/>
   <connectionStrings/>
   <system.web>
      <httpModules>
         <add type="NHibernateHttpModule" name="NHibernateHttpModule"/>
      </httpModules>
      <!--
            Set compilation debug="true" to insert debugging
            symbols into the compiled page. Because this
            affects performance, set this value to true only
            during development.
        -->
      <compilation debug="true">
         <assemblies>
            <add assembly="ICSharpCode.SharpZipLib, Version=0.84.0.0, Culture=neutral, PublicKeyToken=1B03E6ACF1164F73"/>
            <add assembly="System.Data.OracleClient, Version=2.0.0.0, Culture=neutral, PublicKeyToken=B77A5C561934E089"/></assemblies></compilation>
      <!--
            The <authentication> section enables configuration
            of the security authentication mode used by
            ASP.NET to identify an incoming user.
        -->
      <authentication mode="Windows"/>
      <!--
            The <customErrors> section enables configuration
            of what to do if/when an unhandled error occurs
            during the execution of a request. Specifically,
            it enables developers to configure html error pages
            to be displayed in place of a error stack trace.

        <customErrors mode="RemoteOnly" defaultRedirect="GenericErrorPage.htm">
            <error statusCode="403" redirect="NoAccess.htm" />
            <error statusCode="404" redirect="FileNotFound.htm" />
        </customErrors>
        -->
   </system.web>
</configuration>