Can't access Shared Folder of Windows Server 2008 Standard

OS -  Windows Server 2008 Standard 64 Bit , WORKGROUP

Anti-Virus - Symantec Endpoint

- Everyday I have to restart the server to be able to access shared folder. After 4 or 5 hours, all PCs could not access to Server Shared Folder suddenly.

So, every morning I restart the server, and afternoon couldn't access anymore.

What could be the problem?

Thanks.