The RPC over HTTP connection cannot be set up when you try to access certain services on a computer that is running Windows Vista or Windows Server 2008
Consider the following scenario. You have a computer that is running Windows Vista or Windows Server 2008. You configure the computer to use the RPC over HTTP feature to access certain services. In this scenario, the RPC over HTTP module (Rpchttp.dll) may not work correctly. Therefore, the RPC over HTTP connection does not establish successfully. For example, you cannot sign in to Outlook Anywhere or sign in to Office Communicator. Additionally, you are prompted to provide your user credentials.
For more information on this issue, including potential causes, workarounds, and resolutions, see: Microsoft KB Article KB2458828.