The real time antivirus program or the vShield endpoint may cause the vCenter 5.1 service fail to start. I found this out the other day in the vpxd.log file. The error is something like No connection could be made because the target machine actively refused it.
This is due to the real time anti virus scan puts a lock on the files used by the SSO (Single Sign On) or make the file unavailable. The solution is to add the folder C:\Program Files\VMware\Infrastructure\SSOServer\ to the scan exclusion list.
2013-11-21T15:26:13.619+11:00 [05068 error ‘HttpConnectionPool-000001’] [ConnectComplete] Connect failed to <cs p:000000000974cf40, TCP:my-vcenter-01:7444>; cnx: (null), error: class Vmacore::SystemException(No connection could be made because the target machine actively refused it. )
2013-11-21T15:26:13.619+11:00 [03856 error ‘[SSO][SsoCertificateManagerImpl]’] [CreateAdminSsoServiceContent] Failure while trying to connect to SSO Admin server: No connection could be made because the target machine actively refused it. . Will retry in 10 seconds.