The Security event log could not be cleared due to the following error: Access is Denied.
This event is logged when EventSentry cannot clear an event log because of insufficient privileges. The logon account the EventSentry service agent (the service name is listed as EventSentry in Windows Services console) is running under does not have enough privileges to clear the security event log.
The account running under EventSentry service agent can be changed to a domain admin account from Start -> Programs -> Administrative Tools -> Services -> EventSentry -> (right-click Properties) -> Logon As.
You can configure Backup Event Logs feature at Packages -> System Health Packages -> Package Name -> Backup Event Logs.