Connection probs with W2K server

When logging on Windows XP PC’s to Windows 2000 server sometimes the PC’s hang. The mouse responds but there are no icons on desktop and you cannot access the taskbar. The only way to logout is to pull the plug! Sometimes if I wait 10 minutes then the PC will logon. We are using new Dell Optiplex, Dimenson and Latitude PC’s with Etrust Antivirus and a mix of Win XP SP1 and SP2. The Win98 machines dont have this problem – any ideas????