We seen this problem for years but never a solution. We are running Zenworks 7 but have seen this with earlier versions. When people login and the Application Launcher starts, sometimes it will end with the screen frozen and any apps that it would put on the desktop are white. You can usually get by it by Ctrl-Alt-Del, killing explorer.exe and then starting explorer again. My biggest problem is when it happens to students who we hide the task list from. It is very random, may start with new computer image and then only occasionally thereafter.
Any suggestions appreciated.