Security Exception when writting to EventLog

[JMMB]

When trying to execute the following code in a web service, I get the
enclosed error. Everyone has full control do the registry and Security System
configured with the default "Full Control" for all code.
thanks a lot.

WEBMETHOD:
EventLog log = new EventLog("ContosoLog");
log.Source = "MyServiceApplication";
log.WriteEntry("My sessionID", EventLogEntryType.Warning);

ERROR:
System.Web.Services.Protocols.SoapException: Server was unable to process
request. ---> System.Security.SecurityException: Requested registry access is
not allowed. at Microsoft.Win32.RegistryKey.OpenSubKey(String name, Boolean
writable) at System.Diagnostics.EventLog.FindSourceRegistration(String
source, String machineName, Boolean readOnly) at
System.Diagnostics.EventLog.SourceExists(String source, String machineName)
at System.Diagnostics.EventLog.WriteEntry(String message, EventLogEntryType
type, Int32 eventID, Int16 category, Byte[] rawData) at
System.Diagnostics.EventLog.WriteEntry(String message, EventLogEntryType
type, Int32 eventID, Int16 category) at
System.Diagnostics.EventLog.WriteEntry(String message, EventLogEntryType
type, Int32 eventID) at System.Diagnostics.EventLog.WriteEntry(String
message, EventLogEntryType type) at MyService.Service1.SessionID() in
c:\inetpub\wwwroot\myservice\service1.asmx.cs:line 79 --- End of inner
exception stack trace ---