40968LSASRVThe Security System has received an authentication request that could not be decoded. The request has failed.This error is logged when a Windows Server 2003 is scanned by the Nessus scanner.
Event Log Doctor is dead-on. Someone is interested in obtaining root level access to your machine. Check your user list for odd entries, change your administrator password to one that is complex and run netstat in a CLI to look for unusual connections to foreign machines.