Hi Omar,
The latest releases of SEP are fully supported on SBS 2011 so any issue that you are encountereing would not be down to that OS / combination of services.
Support for Symantec Endpoint Protection and Windows Small Business Server 2011 (http://www.symantec.com/docs/TECH156294)
Once the server does fully boot and you are able to log in, are there any errors seen in the Windwos System or Application event logs?
Thanks and best regards,
Mick