I recently ran into a requirements for retrofitting a logging library to an existing project. My first instinct was to throw Pantheios at it as I've used it before and It Just Worked. Unfortunately in this case, we needed the ability to log to more than two event sinks and it looked like this was getting a little awkward with Pantheios, which prompted me to look at Boost.Log.
After some digging through the documentation and the samples, I managed to get the logging going to the three event sinks we needed. So far, so good, but every time I started up the program it reported an unhandled exception on Windows 7 when it was trying to initialise the simple_event_log backend and the software wasn't run as administrator. Curiously enough, the log messages still did appear in the event log, just with lots of unnecessary decoration.
The reason for this problem was that the registry key in HKEY_LOCAL_MACHINESystemCurrentControlSetServicesEventLog that the application needs access to both has to be present (if you're not administrator, you don't have the privileges to create it) and the user who runs the application also needs to be able to both read and write to it. Normally you'd need the installer to create the key as it tends to run with administrator privileges; the installer also needs to set the permissions on the created key to 'Full Control'. Once both the key and the permissions are set correctly, the backend will register OK without any unhandled exceptions.
Unfortunately, if the event log backend can't create the event log registry entry by itself during its initalisation phase, it is also necessary to point the event log at the file that contains the event messages. In order to do this, the installer also needs to create a string value in the newly added application-specific key that has the name "EventMessageFile" and that points at the correct boost_log dll.
Once the above entries are in the registry, logging to the event log using the simple_event_log backend Just Works, too.