Microsoft Knowledge Base Article 910341 contains the following summary:

This article describes how to troubleshoot error messages that you may receive when you try to download updates from the Microsoft Windows Update Web site, from the Microsoft Update Web site, or from a Microsoft Windows Server Updates Services (WSUS) server. To troubleshoot this issue, the "Resolution" section includes steps for Microsoft Windows 2000, for Microsoft Windows Server 2003, and for Microsoft Windows XP. This article also includes a "Workaround" section. You can follow the steps that are in the "Workaround" section if the steps that are in the "Resolution" section do not resolve the problem.