If you have installed Microsoft Exchange Server 5.5 Standard Edition on one of the nodes participating in a Microsoft Cluster Server configuration, you cannot install any service pack (SP) because the SP tries to install itself to the cluster and not to the standard version of Exchange Server.
This affects all servers with Exchange Server that are not using the cluster option but which are running on a cluster server.
This problem occurs if you install Exchange Server 5.5 Standard Edition on a computer that you then upgrade to form a cluster by upgrading the operating system to Windows NT 4.0 Enterprise Edition and Microsoft Cluster Server.
This behavior is by design. If you want to run Exchange Server 5.5 on a cluster node, you need to install the Enterprise Edition of Exchange Server 5.5.