Microsoft Knowledge Base Article 326644 contains the following summary:

The Cluster service may not be able to fail over or to bring a virtual server online. This can occur on an active/passive server cluster on Windows 2000 Advanced Server and may indicate a problem with Domain Name System (DNS) registration. To help you troubleshoot the problem, this article describes the DNS traffic that is typically expected when a virtual server starts. You can compare the expected traffic to traffic that you see on your cluster.

This article also describes one type of DNS failure that can prevent the Cluster service from bringing a cluster virtual server online correctly.