You may receive an error message when accessing the Windows Update v.5 Web site, or when using Automatic Updates. This issue only affects users or Automatic Updates behind a proxy server requiring authentication, or when null user requests are not allowed.
There are two issues affecting Windows Update v.5 when responding to Web proxy requests to access Windows Update:
Issue 1: Credentials may be sent on a half-closed connection. This issue occurs when the client sends a HEAD request on a TCP connection that has already been closed by the proxy server. This is a known Internet Explorer issue, as described in article 838893, "The server returned an invalid or unrecognized response" error message in Internet Explorer 6 Service Pack 1 in the Microsoft Knowledge Base.
Issue 2: Authentication responses may contain null credentials. (Domain and user name are empty.) This issue occurs because the Windows Update client authenticates with null credentials to the proxy server. If the proxy server does not allow null client requests, the request may be denied.
One or both of these behaviors may be observed during a single client session, but not for the same HTTP request.
Symptoms for Issue 1: Credentials may be sent on a half-closed connection and for Issue 2: Authentication responses may contain null credentials are described in the following sections.
This issue may occur when you first connect to the Windows Update site, when the Windows Update component is checking for the latest version of the Windows Update software. A message similar to the following will be displayed, with the error number specified:
Windows Update has encountered an error and cannot display the requested page. In the upper right on the page, you will see the following: [Error number: 0x80072F78].
With this issue, the initial connection to the site will be successful. The error will occur after the check has been made for the Windows Update component updates, when you select Express Install or Custom Install. The following message may be displayed, with the error number specified:
Windows Update has encountered an error and cannot display the requested page. In the upper right on the page, you will see the following: [Error number: 0x80244021].
In addition, the null log on may produce a message with error code 0x80244018 or 0x80244019.
A workaround for configuring ISA Server 204 2004 is described.....
la suite sur : http://www.microsoft.com/technet/prodtechnol/winxppro/support/updateauthen.mspx