The logon process may take a long time when you try to log on to a Windows Vista-based or Windows Server 2008-based computer
When you try to log on to a Windows Vista-based computer or a Windows Server 2008-based computer, the logon process may take a long time because of incremental network traffic.
This problem occurs when you have large number of offline files on the computer.
To determine whether you are experiencing this problem, open a network monitor tool. Then, stop and restart the Windows Search service. If you notice lots of network traffic, you are experiencing this issue.
For more information on this issue, including potential causes, workarounds, and resolutions, see: Microsoft KB Article KB959488.