WINDOWS 2000 SERVER
Begin by making sure you have adequate backups and other safeguards in place so you'll be able to recover data and/or servers in the event of an unforeseen problem with the upgrade process.
When you're ready to upgrade the first node in the cluster, open Cluster Administrator, right-click the node, and choose Pause Node. After pausing the node, right-click it, and choose Move Group. Move all of the groups on the node to another node in the cluster.
After moving the groups, install the service pack on the node, and restart the computer. Check the event logs to identify any potential problems, and address any problems that come up.
After you've properly updated the node, open Cluster Administrator again, and choose Resume Node. Then, move back groups to the node as needed. Repeat this process on other nodes in the cluster one at a time.