If Backup Exec is being installed in a Windows Cluster setup where the various servers involved in the setup are running different versions of Microsoft Windows, then intermittent problems may be observed with Backup Exec functionality. This issue will occur if the mixed setup involves some clustered Backup Exec servers running Microsoft Windows 2008 Operating systems and some clustered Backup Exec servers running older versions of Microsoft Windows 2000/2003/XP.

The issues occur due to a limitation imposed by the OpenCluster API on these versions of Windows. Microsoft documents the limitation as follows: (See the 'REFERENCE:' section below)
Computers running Windows Server 2008 and Windows Vista cannot call OpenCluster API call against a cluster running Windows Server 2003 or Windows 2000 Server, and computers running Windows Server 2003, Windows XP, or Windows 2000 cannot call OpenCluster API call against a cluster running Windows Server 2008.

Listed below are some known problems observed in such a mixed setup:

Backup Exec Central Administration Server (CASO) and Primary Shared Storage Option (SSO) server running on Windows 2003 server with the Backup Exec Database (BEDB) running on a remote clustered SQL 2005/2008 instance  using Windows 2008 Failover Clustering (Clustered nodes Node A and Node B running Windows 2008)

When trying to install Backup Exec along with the MMS option on a Windows 2003 server, the installation may fail.

Backup Exec Central Administration Server (CAS) and Primary Shared Storage Option (SSO) server running on Windows 2003 .

Two clustered nodes running Windows 2008 (Node A and Node B)

After installing Backup Exec as a cluster aware application on Node A and Node B, when the MMS/SAN-SSO option is installed in the clustered Backup Exec installation, the Backup Exec Cluster Virtual Node name may not appear on the Central Administration Server (CASO) Media server tab or appears offline or unmanageable.   The individual cluster node names may show up offline or disappear from the CASO servers media server tab.

Note: The same problem may even occur in the reverse scenario where CAS server is running Windows 2008 and the MMS is clustered on nodes running Windows 2003 .

Backup Exec Central Administration Server (CAS) and Primary Shared Storage Option (SSO) server running on Windows 2003 .

Two clustered nodes running Windows 2008 (Node A and Node B)

Backup Exec is installed on the Node A and Node B separately. The Backup Exec Managed Media Server (MMS) option is added to Cluster Node A and Node B individually. Each node is now a (MMS) server.

The Backup Exec SSO License is added to Cluster Node A and Node B making each node a Secondary SSO server.

Cluster Node A and Node B now get displayed on the Media Server Tab on the Windows 2003 CASO server.

Now on Cluster Node A the Backup Exec Cluster Configuration Wizard is run to create a Backup Exec Cluster. However After the Cluster wizard completes the following issues may be observed:

Note: The same problem may even occur in the reverse scenario where CAS server is running Windows 2008 and the MMS is clustered on nodes running Windows 2003 .

Scenario 4:
Backup Exec is being installed on a Windows 2003 stand alone server. User selects during the install, to use a clustered SQL Server instance running on Windows 2008. The installation will fail.

Scenario 5:
Backup Exec is being installed on a Windows 2008 stand alone server. User selects during the install, to use a clustered SQL Server instance running on Windows 2003. The installation will fail.

When Backup Exec is configured to use Microsoft Windows Failover Clustering, all of the servers must be hosted on the same cluster compatible platform. For example:
If the Backup Exec Server is clustered on Windows Server 2003 and below, then CAS, SAN-SSO servers must also be running Windows Server 2003 or below.
If the Backup Exec Server is clustered on Windows Server 2008 and above, then CAS, SAN-SSO servers must also be running Windows Server 2008 or above.

Also:
If Backup Exec needs to be installed on a Windows Server 2003 or below, with the Backup Exec Database being hosted on a clustered remote SQL server instance then it too, must be running on Windows Server 2003 and below.
If Backup Exec needs to be installed on a Windows Server 2008 or above, with the Backup Exec Database being hosted on a clustered remote SQL server instance, it too, must be running on Windows Server 2008 and above.

MSDN OpenCluster Function: http://msdn.microsoft.com/en-us/library/aa371774(VS.85).aspx