Microsoft Knowledge Base Article 908370 contains the following summary:
A domain controller that is running Microsoft Windows Server 2003 may stop responding for 2 to 15 minutes several times a day.
A Server Message Block (SMB) session may be unresponsive when the user of a client computer tries to browse files or to open files on a network. The problem affects all the client computers at the same time. The problem may also occur when you try to browse files on the server by using a Universal Naming Convention (UNC) path.
When this problem occurs, you can view a shared location on the network by using a Command Prompt window. The server uses no more resources than usual.