Setup detected that the Cluster Service is installed. To properly upgrade a Windows NT 4.0 Server Cluster, you must upgrade one node at a time with the other node powered off (rolling upgrade is not supported). To properly upgrade a Windows 2000 Server Cluster node, the Cluster Service must be running on that node.  All nodes in the Cluster must be Windows 2000 or later to maintain interoperability in a mixed-version Server Cluster. 

See the Getting Started manual under "Upgrading and Installing on Cluster Nodes" for more information.