In September 2011 Enterprise Vault adopted a new Digital Certificate for signing files. This certificate's root certificate is Verisign Class 3 Public Primary Certification Authority G5. Most systems that have an internet connection will get this certificate via an automatic update, but if that doesn't happen then you will get an error like this in the installation log when you attempt to install the FSA agent on that system:
DIFXAPP: ERROR: Signature verification
failed while checking integrity of driver package 'evmf.inf'
If you check the MSI package's digital signature on the target file server, you will see that the root certificate cannot be found on the local system.
To address the problem you will need to install the new root certificate on the file server. For more details about how to obtain the certificate, see the following Microsoft KB article: http://support.microsoft.com/kb/931125.