If you have an access denied when writing to the event log, the user probably doesn't have or has lost the right to write to the event log. The solution is to give him the right to write to the event log for the authenticated user.   

In regedit navigate to  [HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Eventlog\Application] for example

Append (A;;0x3;;;AU) to the customSd Key, that will the right to Authenticated User (AU) to write to the chosen event log 

Tags: windows 2003