Have two new Windows 2008 SP2 fileservers on our customers site, and about 550 Windows XP SP3 clients that have mapped drives to these servers. The servers ran fine for three weeks. We started then to migrate data from old Windows 2003 servers to these new file servers. Since then, the Windows XP clients loose completely the connection to the Windows 2008 servers, about once or twice a day. - Rebooting the file servers solves the problem. - What is completely astonishing: Windows Vista and Windows 7 clients NEVER loose the connection to the Windows 2008 file servers. Only Windows XP and Windows 2003 servers as well are affected- Have change the NIC's in the server (HP G6, changed from broadcom to intel NIC's)- Have completely disabled all TCP offload functions (on the NIC, and according KB 951037). Have the impression that this action has increased the times between the server failures, but they still occur.- disabled windows firewall on the Windows 2008 servers- there are absolutely no messages in the server- or client eventlogs that would indicate a trace to the problem- There are no error messages on the XP client when the problem arise for 5 minutes at least (we cannot wait longer, we have to reboot the server to re-establish access for the users). The Windows explorer nearly hangs, but with 0% CPU utilization. When trying to access the server share from the command prompt, I have seen one time an "system error 64 - specified network name is no longer available" error message.- Would suspect network errors, but all the productive Windows 2003 file servers connected to the same network are never affected by this problem.We have opened a Microsoft PSS Call for this two days ago. But as a little Microsoft Partner, you have to go throug some kind of first level support and according our experience, it takes sometimes weeks until the case is escalated to the guys that really understand the problem and are able to help.We are lost, should migrate data to the new servers since weeks (and can not do that because of this problem), and looking for any help that we can get, and looking forward for any hint or help we can get. Thank you in advance!Franz

Try enabling the computer browser service on the 2008 servers. It is disabled by default. The fact that the Vista and 7 clients do not lose connections makes me think that they are communicating with the 2008 servers using ipv6 neighbor discovery.Bits of Fury

Thank you for your answer. But IPv6 is already disabled on the Windows 2008 server, according KB http://support.microsoft.com/kb/929852Franz

After days of research, I have found the cause of the problem, it's well known at Microsoft since July, 2008: http://blogs.technet.com/networking/archive/2008/07/10/intermittent-file-sharing-connectivity-from-various-clients-to-a-windows-server-2008-server.aspx. I'm a bit disappointed that no one from Microsoft PSS nor in this forum knows about that issue. The culprit is Symantec AV Software (Client Security, V10). Have removed this SW, and the problem is solved.Franz