Issue with Network Session\Share Folder

In the last week I have started to experience a number of issues arising on a Win 2003 R2 fully patched server.
To start with some of the users drive mappings were timing out (small red cross) no problem there as when they clicked the shared was reconnected.
However, I now have the issue where it does not reconnect as states there are no available sessions? I logon the server kill some sessions and they can then connect. All workstations are XP Pro SP2. There is nothing in either events logs on the serevr or WS?
Any ideas would be appreciated. If you need more info please ask.