Hi,
Try restarting the machine in safe mode with networking. This would load the Symantec and the Microsoft files needed for startup. Then try to log off and check whether you face the same issue. If yes the issue is either related to Symantec or the core OS. In this case try uninstalling endpoint and then check whether the issue is resolved ..in case not then the issue is with the OS.
If you are able to log off successfully in Safe mode with Networking then start the machine in Normal mode...run the MSCONFIG utility (in case you are comfortable using it) to find the service and the process that is causing this issue. Please note that you need to login as an Administrator to make the changes. In case of any doubts about the MSCONFIG utility please refer the below document
http://support.microsoft.com/kb/310560
Thanks
Sandip