Microsoft Knowledge Base Article 269732 contains the following summary:
This article discusses how to obtain the Windows Installer package file (Update.msi) for Windows 2000 service packs.
• Using Windows Installer Service, the Windows Installer Package (Update.msi) allows you to streamline the deployment and maintenance of service packs from a network.
• The Windows Installer package file (Update.msi) contains all of the information that is required to install or uninstall the service pack by using the IntelliMirror management technologies in Windows 2000.
• Microsoft recommends the Update.msi package for system administrators and other advanced users.
• This installation method assumes that you have a basic understanding of the Windows Installer service, and a working knowledge of Group Policy and Active Directory.