Microsoft Knowledge Base Article 271135 contains the following summary:

This article discusses the Microsoft Management Console (MMC) and snap-in restrictions for a Microsoft Windows 2000-based computer.                              When you attempt to run an MMC snap-in, you may receive the following error message:                                   The snap-in below, referenced in this document has been restricted by policy.                                  Contact your administrator for details. snap-in name                              Or, MMC starts, but the list of available snap-ins is missing or limited.