Detecting User logon/Logoff events for a Windows Service

Question

I have written a windows service that needs to perform certain functions only if a User is logged in. The problem I am running into is that I get an error if the computer is in the process of being shut down or restarted. At all other times, the service runs just fine. I need to either detect a logon/logoff event OR check somehow as to whether there is currently a valid logged on user. The closest I have found is ManagementEventWatcher class, but I am not able to determine the appropiate event class to query for the Audit logon/logoff events. Basically I just need to not try and open the ad-watch if the user has logged off (Event Id 538). Of course, if there is an easier check, please let me know of that! I do appreciate any assistance on this. Thank you!