Windows XP Blank Desktop at Login

I have several users/PCs that get a blank desktop when logging on our Novell 5.1 network. It seems only to be a problem with Windows XP on several Dell GX50 Celeron PCs – all within a year old. I have tried deleting local user profiles, restore points, rebuilding the PC, etc., but the problem keeps coming back intermittently. I have found a workaround ? at the blank desktop, the user can do a CTRL-ALT-DEL and run a new task called Explorer.exe which fully brings up the desktop. Does this workaround give any clues to the problem? Either way, I am looking for a permanent fix which does not involve rebuilding the PC every time ? which does not last for long as a solution anyway. Please help if you can. Thanks.