<system.web>
        <customErrors mode="Off" />
    </system.web>
    <system.webServer>
        <httpErrors errorMode="Detailed" />
    </system.webServer>

using System;
using System.Web;

namespace HttpModules
{
    public class ErrorModule: IHttpModule
    {
        public void Init(HttpApplication context)
        {
            context.Error += ProcessError;
        }

        private void ProcessError(object sender, EventArgs e)
        {
            Exception ex = HttpContext.Current.Server.GetLastError();
            HttpContext context = HttpContext.Current;
            if (ex is HttpException)
            {
                switch ((ex as HttpException).GetHttpCode())
                {
                    case 404:
                        {
                            context.Response.StatusCode = 404;
                            context.Response.StatusDescription = "Not Found";
                            context.Response.Charset = "windows-1251";
                            context.Server.Execute("~/FileNotFound.html");

                            context.Server.ClearError();

                            break;
                        }
                }

            }
        }

        public void Dispose()
        {
            throw new NotImplementedException();
        }
    }
}

<system.web>
...
    <httpModules>
	<add name="ErrorModule" type="HttpModules.ErrorModule, HttpModules" />
    </httpModules>
</system.web>

<system.webServer>
...
    <httpErrors errorMode="Custom" existingResponse="Replace">
      <clear />
      <error statusCode="404" prefixLanguageFilePath="" path="/FileNotFound.html" responseMode="ExecuteURL" />
    </httpErrors>
</system.webServer>

using System.Diagnostics;
using System.Runtime.CompilerServices;
using System.Web;
using System.Web.Profile;

namespace ASP
{
  [CompilerGlobalScope]
  public class global_asax : HttpApplication
  {
    private static bool __initialized;

    public int ProductId
    {
      get
      {
        int num = 0;
        if (this.Request.QueryString["id"] != null)
        {
          try
          {
            num = Convert.ToInt32(this.Request.QueryString["id"]);
          }
          catch
          {
            num = 0;
          }
        }
        return num;
      }
    }

    protected DefaultProfile Profile
    {
      get
      {
        return (DefaultProfile) this.Context.Profile;
      }
    }

    [DebuggerNonUserCode]
    public global_asax()
    {
      if (global_asax.__initialized)
        return;
      global_asax.__initialized = true;
    }

    protected void Application_BeginRequest(object sender, EventArgs e)
    {
      try
      {
        if (this.ProductId <= 0)
          return;
        this.Response.AddHeader("Location", SiteUrls.ProductServiceName(this.ProductId));
        this.Response.Status = "301 Moved Permanently";
      }
      catch (ProductIdNotFoundException ex)
      {
        this.Response.TrySkipIisCustomErrors = true;
        this.Response.Status = "404 Not Found";
        this.Response.StatusCode = 404;
        this.Server.Transfer("~/FileNotFound.html");
      }
    }

    private void Application_Start(object sender, EventArgs e)
    {
    }

    private void Application_End(object sender, EventArgs e)
    {
    }

    private void Application_Error(object sender, EventArgs e)
    {
        HttpContext oHttpContext;
        Exception oException;

        oHttpContext = HttpContext.Current;

        oException = oHttpContext.Server.GetLastError();

        if (oException is HttpException)
        {
            switch ((oException as HttpException).GetHttpCode())
            {
                case 404:

                    oHttpContext.Response.StatusCode = 404;
                    oHttpContext.Response.StatusDescription = "Not Found";
                    oHttpContext.Response.Charset = "windows-1251";
                    oHttpContext.Server.Execute("~/FileNotFound.html");

                    oHttpContext.Server.ClearError();

                    break;
            }
        }
    }

    private void Session_Start(object sender, EventArgs e)
    {
      this.Session["ShoppingCart"] = (object) new ShoppingCart();
    }

    private void Session_End(object sender, EventArgs e)
    {
    }
  }
}